diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/utils/FHIRPathEngine.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/utils/FHIRPathEngine.java
index e35d08127..939d93026 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/utils/FHIRPathEngine.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/utils/FHIRPathEngine.java
@@ -2903,6 +2903,8 @@ public class FHIRPathEngine {
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Quantity);
     } else if (constant instanceof FHIRConstant) {
       return resolveConstantType(context, ((FHIRConstant) constant).getValue(), expr);
+    } else if (constant == null) {
+      return new TypeDetails(CollectionStatus.SINGLETON);      
     } else {
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String);
     }
@@ -3140,60 +3142,60 @@ public class FHIRPathEngine {
       return types;
     }
     case Lower : {
-      checkContextString(focus, "lower", exp);
+      checkContextString(focus, "lower", exp, true);
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String); 
     }
     case Upper : {
-      checkContextString(focus, "upper", exp);
+      checkContextString(focus, "upper", exp, true);
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String); 
     }
     case ToChars : {
-      checkContextString(focus, "toChars", exp);
+      checkContextString(focus, "toChars", exp, true);
       return new TypeDetails(CollectionStatus.ORDERED, TypeDetails.FP_String); 
     }
     case IndexOf : {
-      checkContextString(focus, "indexOf", exp);
+      checkContextString(focus, "indexOf", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Integer); 
     }
     case Substring : {
-      checkContextString(focus, "subString", exp);
+      checkContextString(focus, "subString", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Integer), new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Integer)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String); 
     }
     case StartsWith : {
-      checkContextString(focus, "startsWith", exp);
+      checkContextString(focus, "startsWith", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Boolean); 
     }
     case EndsWith : {
-      checkContextString(focus, "endsWith", exp);
+      checkContextString(focus, "endsWith", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Boolean); 
     }
     case Matches : {
-      checkContextString(focus, "matches", exp);
+      checkContextString(focus, "matches", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Boolean); 
     }
     case MatchesFull : {
-      checkContextString(focus, "matches", exp);
+      checkContextString(focus, "matches", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Boolean); 
     }
     case ReplaceMatches : {
-      checkContextString(focus, "replaceMatches", exp);
+      checkContextString(focus, "replaceMatches", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String), new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String); 
     }
     case Contains : {
-      checkContextString(focus, "contains", exp);
+      checkContextString(focus, "contains", exp, true);
       checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_Boolean);
     }
     case Replace : {
-      checkContextString(focus, "replace", exp);
-      checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, "string"), new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
+      checkContextString(focus, "replace", exp, true);
+      checkParamTypes(exp, exp.getFunction().toCode(), paramTypes, new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String), new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String)); 
       return new TypeDetails(CollectionStatus.SINGLETON, TypeDetails.FP_String);
     }
     case Length : { 
@@ -3427,37 +3429,39 @@ public class FHIRPathEngine {
   }
 
 
-  private void checkContextString(TypeDetails focus, String name, ExpressionNode expr) throws PathEngineException {
-    if (!focus.hasType(worker, "string") && !focus.hasType(worker, "code") && !focus.hasType(worker, "uri") && !focus.hasType(worker, "canonical") && !focus.hasType(worker, "id")) {
-      throw makeException(expr, I18nConstants.FHIRPATH_STRING_ONLY, name, focus.describe());
+  private void checkContextString(TypeDetails focus, String name, ExpressionNode expr, boolean sing) throws PathEngineException {
+    if (!focus.hasNoTypes() && !focus.hasType(worker, "string") && !focus.hasType(worker, "code") && !focus.hasType(worker, "uri") && !focus.hasType(worker, "canonical") && !focus.hasType(worker, "id")) {
+      throw makeException(expr, sing ? I18nConstants.FHIRPATH_STRING_SING_ONLY : I18nConstants.FHIRPATH_STRING_ORD_ONLY, name, focus.describe());
     }
   }
 
 
   private void checkContextPrimitive(TypeDetails focus, String name, boolean canQty, ExpressionNode expr) throws PathEngineException {
-    if (canQty) {
-      if (!focus.hasType(primitiveTypes) && !focus.hasType("Quantity")) {
-        throw makeException(expr, I18nConstants.FHIRPATH_PRIMITIVE_ONLY, name, focus.describe(), "Quantity, "+primitiveTypes.toString());
+    if (!focus.hasNoTypes()) {
+      if (canQty) {
+        if (!focus.hasType(primitiveTypes) && !focus.hasType("Quantity")) {
+          throw makeException(expr, I18nConstants.FHIRPATH_PRIMITIVE_ONLY, name, focus.describe(), "Quantity, "+primitiveTypes.toString());
+        }
+      } else if (!focus.hasType(primitiveTypes)) {
+        throw makeException(expr, I18nConstants.FHIRPATH_PRIMITIVE_ONLY, name, focus.describe(), primitiveTypes.toString());
       }
-    } else if (!focus.hasType(primitiveTypes)) {
-      throw makeException(expr, I18nConstants.FHIRPATH_PRIMITIVE_ONLY, name, focus.describe(), primitiveTypes.toString());
     }
   }
 
   private void checkContextNumerical(TypeDetails focus, String name, ExpressionNode expr) throws PathEngineException {
-    if (!focus.hasType("integer")  && !focus.hasType("decimal") && !focus.hasType("Quantity")) {
+    if (!focus.hasNoTypes() && !focus.hasType("integer")  && !focus.hasType("decimal") && !focus.hasType("Quantity")) {
       throw makeException(expr, I18nConstants.FHIRPATH_NUMERICAL_ONLY, name, focus.describe());
     }    
   }
 
   private void checkContextDecimal(TypeDetails focus, String name, ExpressionNode expr) throws PathEngineException {
-    if (!focus.hasType("decimal") && !focus.hasType("integer")) {
+    if (!focus.hasNoTypes() && !focus.hasType("decimal") && !focus.hasType("integer")) {
       throw makeException(expr, I18nConstants.FHIRPATH_DECIMAL_ONLY, name, focus.describe());
     }    
   }
 
   private void checkContextContinuous(TypeDetails focus, String name, ExpressionNode expr) throws PathEngineException {
-    if (!focus.hasType("decimal") && !focus.hasType("date") && !focus.hasType("dateTime") && !focus.hasType("time")) {
+    if (!focus.hasNoTypes() && !focus.hasType("decimal") && !focus.hasType("date") && !focus.hasType("dateTime") && !focus.hasType("time")) {
       throw makeException(expr, I18nConstants.FHIRPATH_CONTINUOUS_ONLY, name, focus.describe());
     }    
   }
@@ -3672,7 +3676,10 @@ public class FHIRPathEngine {
 
 
   private List<Base> funcExp(ExecutionContext context, List<Base> focus, ExpressionNode expr) {
-    if (focus.size() != 1) {
+    if (focus.size() == 0) {
+      return new ArrayList<Base>();
+    }
+    if (focus.size() > 1) {
       throw makeException(expr, I18nConstants.FHIRPATH_FOCUS_PLURAL, "exp", focus.size());
     }
     Base base = focus.get(0);
@@ -4218,15 +4225,19 @@ public class FHIRPathEngine {
 
   private List<Base> funcReplace(ExecutionContext context, List<Base> focus, ExpressionNode expr) throws FHIRException, PathEngineException {
     List<Base> result = new ArrayList<Base>();
+    List<Base> tB = execute(context, focus, expr.getParameters().get(0), true);
+    String t = convertToString(tB);
+    List<Base> rB = execute(context, focus, expr.getParameters().get(1), true);
+    String r = convertToString(rB);
 
-    if (focus.size() == 1) {
+    if (focus.size() == 0 || tB.size() == 0 || rB.size() == 0) {
+      //
+    } else if (focus.size() == 1) {
       if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
         String f = convertToString(focus.get(0));
         if (Utilities.noString(f)) {
           result.add(new StringType(""));
         } else {
-          String t = convertToString(execute(context, focus, expr.getParameters().get(0), true));
-          String r = convertToString(execute(context, focus, expr.getParameters().get(1), true));
           String n = f.replace(t, r);
           result.add(new StringType(n));
         }
@@ -4240,10 +4251,14 @@ public class FHIRPathEngine {
 
   private List<Base> funcReplaceMatches(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
-    String regex = convertToString(execute(context, focus, exp.getParameters().get(0), true));
-    String repl = convertToString(execute(context, focus, exp.getParameters().get(1), true));
+    List<Base> regexB = execute(context, focus, exp.getParameters().get(0), true);
+    String regex = convertToString(regexB);
+    List<Base> replB = execute(context, focus, exp.getParameters().get(1), true);
+    String repl = convertToString(replB);
 
-    if (focus.size() == 1 && !Utilities.noString(regex)) {
+    if (focus.size() == 0 || regexB.size() == 0 || replB.size() == 0) {
+      //
+    } else if (focus.size() == 1 && !Utilities.noString(regex)) {
       if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
         result.add(new StringType(convertToString(focus.get(0)).replaceAll(regex, repl)).noExtensions());
       }
@@ -4256,10 +4271,13 @@ public class FHIRPathEngine {
 
   private List<Base> funcEndsWith(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
-    String sw = convertToString(execute(context, focus, exp.getParameters().get(0), true));
+    List<Base> swb = execute(context, focus, exp.getParameters().get(0), true);
+    String sw = convertToString(swb);
 
     if (focus.size() == 0) {
-      result.add(new BooleanType(false).noExtensions());
+      //
+    } else if (swb.size() == 0) {
+      //
     } else if (Utilities.noString(sw)) {
       result.add(new BooleanType(true).noExtensions());
     } else if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
@@ -4929,9 +4947,12 @@ public class FHIRPathEngine {
 
   private List<Base> funcMatches(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
-    String sw = convertToString(execute(context, focus, exp.getParameters().get(0), true));
+    List<Base> swb = execute(context, focus, exp.getParameters().get(0), true);
+    String sw = convertToString(swb);
 
-    if (focus.size() == 1 && !Utilities.noString(sw)) {
+    if (focus.size() == 0 || swb.size() == 0) {
+      //
+    } else if (focus.size() == 1 && !Utilities.noString(sw)) {
       if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
         String st = convertToString(focus.get(0));
         if (Utilities.noString(st)) {
@@ -4973,10 +4994,13 @@ public class FHIRPathEngine {
 
   private List<Base> funcContains(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
-    String sw = convertToString(execute(context, baseToList(context.thisItem), exp.getParameters().get(0), true));
+    List<Base> swb = execute(context, baseToList(context.thisItem), exp.getParameters().get(0), true);
+    String sw = convertToString(swb);
 
     if (focus.size() != 1) {
-      result.add(new BooleanType(false).noExtensions());
+      //
+    } else if (swb.size() != 1) {
+        //
     } else if (Utilities.noString(sw)) {
       result.add(new BooleanType(true).noExtensions());
     } else if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
@@ -5018,10 +5042,13 @@ public class FHIRPathEngine {
 
   private List<Base> funcStartsWith(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
-    String sw = convertToString(execute(context, focus, exp.getParameters().get(0), true));
+    List<Base> swb = execute(context, focus, exp.getParameters().get(0), true);
+    String sw = convertToString(swb);
 
     if (focus.size() == 0) {
-      result.add(new BooleanType(false).noExtensions());
+      // no result
+    } else if (swb.size() == 0) {
+      // no result
     } else if (Utilities.noString(sw)) {
       result.add(new BooleanType(true).noExtensions());
     } else if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {
@@ -5071,9 +5098,12 @@ public class FHIRPathEngine {
   private List<Base> funcIndexOf(ExecutionContext context, List<Base> focus, ExpressionNode exp) throws FHIRException {
     List<Base> result = new ArrayList<Base>();
 
-    String sw = convertToString(execute(context, focus, exp.getParameters().get(0), true));
+    List<Base> swb = execute(context, focus, exp.getParameters().get(0), true);
+    String sw = convertToString(swb);
     if (focus.size() == 0) {
-      result.add(new IntegerType(0).noExtensions());
+      // no result
+    } else if (swb.size() == 0) {
+      // no result
     } else if (Utilities.noString(sw)) {
       result.add(new IntegerType(0).noExtensions());
     } else if (focus.get(0).hasType(FHIR_TYPES_STRING) || doImplicitStringConversion) {