diff --git a/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu2Test.java b/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu2Test.java
new file mode 100644
index 00000000000..646fd0c3bd4
--- /dev/null
+++ b/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu2Test.java
@@ -0,0 +1,108 @@
+package ca.uhn.fhir.parser;
+
+import ca.uhn.fhir.context.FhirContext;
+import ca.uhn.fhir.model.primitive.StringDt;
+import ca.uhn.fhir.util.TestUtil;
+import org.junit.AfterClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Created by Sébastien Rivière 12/04/2017
+ */
+public class ElementWithExtensionDstu2Test {
+
+	private static final org.slf4j.Logger ourLog = org.slf4j.LoggerFactory.getLogger(ca.uhn.fhir.parser.ElementWithExtensionDstu2Test.class);
+	private static final FhirContext ctx = FhirContext.forDstu2();
+
+	@AfterClass
+	public static void afterClassClearContext() {
+		TestUtil.clearAllStaticFieldsForUnitTest();
+	}
+
+	@Test
+	public void testExtensionOnPrimitiveExtensionJson() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getPetName().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+		final String json = parser.encodeResourceToString(patient);
+
+		ourLog.info(json);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+		assertEquals(1, patient.getPetName().getUndeclaredExtensions().size());
+	}
+
+	@Test
+	public void testExtensionOnPrimitiveExtensionXml() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getPetName().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+		final String xml = parser.encodeResourceToString(patient);
+
+		ourLog.info(xml);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+		assertEquals(1, patient.getPetName().getUndeclaredExtensions().size());
+	}
+
+	@Test
+	public void testExtensionOnIDDatatypeJson() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getId().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+		final String json = parser.encodeResourceToString(patient);
+
+		ourLog.info(json);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+		assertEquals(1, patient.getId().getUndeclaredExtensions().size());
+	}
+
+	@Test
+	public void testExtensionOnIDDatatypeXml() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getId().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+		final String xml = parser.encodeResourceToString(patient);
+
+		ourLog.info(xml);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+		assertEquals(1, patient.getId().getUndeclaredExtensions().size());
+	}
+
+	@Test
+	public void testExtensionOnIDDatatypeExtensionJson() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getCustomId().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+		final String json = parser.encodeResourceToString(patient);
+
+		ourLog.info(json);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+		assertEquals(1, patient.getCustomId().getUndeclaredExtensions().size());
+	}
+
+	@Test
+	public void testExtensionOnIDDatatypeExtensionXml() throws Exception {
+		MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+		patient.setId("1");
+		patient.getCustomId().addUndeclaredExtension(false, "http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringDt("UNK"));
+		final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+		final String xml = parser.encodeResourceToString(patient);
+
+		ourLog.info(xml);
+
+		patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+		assertEquals(1, patient.getCustomId().getUndeclaredExtensions().size());
+	}
+}
+
diff --git a/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java b/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
index 4b62f4ea575..d6aa14058ab 100644
--- a/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
+++ b/hapi-fhir-structures-dstu2/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
@@ -5,6 +5,7 @@ import ca.uhn.fhir.model.api.annotation.Description;
 import ca.uhn.fhir.model.api.annotation.Extension;
 import ca.uhn.fhir.model.api.annotation.ResourceDef;
 import ca.uhn.fhir.model.dstu2.resource.Patient;
+import ca.uhn.fhir.model.primitive.IdDt;
 import ca.uhn.fhir.model.primitive.StringDt;
 
 @ResourceDef()
@@ -17,11 +18,19 @@ public class MyPatientWithCustomUrlExtension extends Patient {
 	@Description(shortDefinition = "The name of the patient's favourite pet")
 	private StringDt myPetName;
 
+	@Child(name = "customid")
+	@Extension(url = "/customid", definedLocally = false, isModifier = false)
+	@Description(shortDefinition = "The customid of the patient's ")
+	private IdDt myCustomId;
+
 	public StringDt getPetName() {
+		if (myPetName == null) {
+			myPetName = new StringDt();
+		}
 		return myPetName;
 	}
 
-	public void setPetName(StringDt thePetName) {
+	public void setPetName(final StringDt thePetName) {
 		myPetName = thePetName;
 	}
 
@@ -30,4 +39,14 @@ public class MyPatientWithCustomUrlExtension extends Patient {
 		return super.isEmpty() && myPetName.isEmpty();
 	}
 
+	public IdDt getCustomId() {
+		if (myCustomId == null) {
+			myCustomId = new IdDt();
+		}
+		return myCustomId;
+	}
+
+	public void setCustomId(final IdDt myCustomId) {
+		this.myCustomId = myCustomId;
+	}
 }
diff --git a/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu3Test.java b/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu3Test.java
new file mode 100644
index 00000000000..516a459b564
--- /dev/null
+++ b/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/ElementWithExtensionDstu3Test.java
@@ -0,0 +1,108 @@
+package ca.uhn.fhir.parser;
+
+import ca.uhn.fhir.context.FhirContext;
+import ca.uhn.fhir.util.TestUtil;
+import org.hl7.fhir.dstu3.model.StringType;
+import org.junit.AfterClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Created by Sébastien Rivière 12/04/2017
+ */
+public class ElementWithExtensionDstu3Test {
+
+  private static final org.slf4j.Logger ourLog = org.slf4j.LoggerFactory.getLogger(ca.uhn.fhir.parser.ElementWithExtensionDstu3Test.class);
+  private static final FhirContext ctx = FhirContext.forDstu3();
+
+  @AfterClass
+  public static void afterClassClearContext() {
+    TestUtil.clearAllStaticFieldsForUnitTest();
+  }
+
+  @Test
+  public void testNullFlavorPrimitiveExtensionJson() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getPetName().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+    final String json = parser.encodeResourceToString(patient);
+
+    ourLog.info(json);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+    assertEquals(1, patient.getPetName().getExtension().size());
+  }
+
+  @Test
+  public void testNullFlavorPrimitiveExtensionXml() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getPetName().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+    final String xml = parser.encodeResourceToString(patient);
+
+    ourLog.info(xml);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+    assertEquals(1, patient.getPetName().getExtension().size());
+  }
+
+  @Test
+  public void testNullFlavorIDDatatypeJson() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getIdElement().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+    final String json = parser.encodeResourceToString(patient);
+
+    ourLog.info(json);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+    assertEquals(1, patient.getIdElement().getExtension().size());
+  }
+
+  @Test
+  public void testNullFlavorIDDatatypeXml() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getIdElement().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+    final String xml = parser.encodeResourceToString(patient);
+
+    ourLog.info(xml);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+    assertEquals(1, patient.getIdElement().getExtension().size());
+  }
+
+  @Test
+  public void testNullFlavorExtensionIDDatatypeJson() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getCustomId().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newJsonParser().setPrettyPrint(true);
+    final String json = parser.encodeResourceToString(patient);
+
+    ourLog.info(json);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, json);
+    assertEquals(1, patient.getCustomId().getExtension().size());
+  }
+
+  @Test
+  public void testNullFlavorExtensionIDDatatypeXml() throws Exception {
+    MyPatientWithCustomUrlExtension patient = new MyPatientWithCustomUrlExtension();
+    patient.setId("1");
+    patient.getCustomId().addExtension("http://hl7.org/fhir/StructureDefinition/iso21090-nullFlavor", new StringType("UNK"));
+    final IParser parser = ctx.newXmlParser().setPrettyPrint(true);
+    final String xml = parser.encodeResourceToString(patient);
+
+    ourLog.info(xml);
+
+    patient = parser.parseResource(MyPatientWithCustomUrlExtension.class, xml);
+    assertEquals(1, patient.getCustomId().getExtension().size());
+  }
+}
+
diff --git a/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java b/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
index d668ecf7e08..cca4b7ede6d 100644
--- a/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
+++ b/hapi-fhir-structures-dstu3/src/test/java/ca/uhn/fhir/parser/MyPatientWithCustomUrlExtension.java
@@ -4,30 +4,49 @@ import ca.uhn.fhir.model.api.annotation.Child;
 import ca.uhn.fhir.model.api.annotation.Description;
 import ca.uhn.fhir.model.api.annotation.Extension;
 import ca.uhn.fhir.model.api.annotation.ResourceDef;
-import org.hl7.fhir.dstu3.model.StringType;
+import org.hl7.fhir.dstu3.model.IdType;
 import org.hl7.fhir.dstu3.model.Patient;
+import org.hl7.fhir.dstu3.model.StringType;
 
 @ResourceDef()
 public class MyPatientWithCustomUrlExtension extends Patient {
 
-    private static final long serialVersionUID = 1L;
+  private static final long serialVersionUID = 1L;
 
-    @Child(name = "petName")
-    @Extension(url = "/petname", definedLocally = false, isModifier = false)
-    @Description(shortDefinition = "The name of the patient's favourite pet")
-    private StringType myPetName;
+  @Child(name = "petName")
+  @Extension(url = "/petname", definedLocally = false, isModifier = false)
+  @Description(shortDefinition = "The name of the patient's favourite pet")
+  private StringType myPetName;
 
-    public StringType getPetName() {
-        return myPetName;
+  @Child(name = "customid")
+  @Extension(url = "/customid", definedLocally = false, isModifier = false)
+  @Description(shortDefinition = "The customid of the patient's ")
+  private IdType myCustomId;
+
+  public StringType getPetName() {
+    if (myPetName == null) {
+      myPetName = new StringType();
     }
+    return myPetName;
+  }
 
-    public void setPetName(final StringType thePetName) {
-        myPetName = thePetName;
-    }
-
-    @Override
-    public boolean isEmpty() {
-        return super.isEmpty() && myPetName.isEmpty();
+  public void setPetName(final StringType thePetName) {
+    myPetName = thePetName;
+  }
+
+  @Override
+  public boolean isEmpty() {
+    return super.isEmpty() && myPetName.isEmpty();
+  }
+
+  public IdType getCustomId() {
+    if (myCustomId == null) {
+      myCustomId = new IdType();
     }
+    return myCustomId;
+  }
 
+  public void setCustomId(final IdType myCustomId) {
+    this.myCustomId = myCustomId;
+  }
 }