diff --git a/modules/lang-painless/src/main/java/org/elasticsearch/painless/MethodWriter.java b/modules/lang-painless/src/main/java/org/elasticsearch/painless/MethodWriter.java
index a982b889194..01eb28fa4d4 100644
--- a/modules/lang-painless/src/main/java/org/elasticsearch/painless/MethodWriter.java
+++ b/modules/lang-painless/src/main/java/org/elasticsearch/painless/MethodWriter.java
@@ -488,4 +488,12 @@ public final class MethodWriter extends GeneratorAdapter {
         }
     }
 
+    @Override
+    public void visitEnd() {
+        if (stringConcatArgs != null && !stringConcatArgs.isEmpty()) {
+            throw new IllegalStateException("String concat bytecode not completed.");
+        }
+        super.visitEnd();
+    }
+
 }
diff --git a/modules/lang-painless/src/test/java/org/elasticsearch/painless/StringTests.java b/modules/lang-painless/src/test/java/org/elasticsearch/painless/StringTests.java
index 8d22323d5b3..e61541bf371 100644
--- a/modules/lang-painless/src/test/java/org/elasticsearch/painless/StringTests.java
+++ b/modules/lang-painless/src/test/java/org/elasticsearch/painless/StringTests.java
@@ -19,6 +19,8 @@
 
 package org.elasticsearch.painless;
 
+import static org.elasticsearch.painless.WriterConstants.MAX_INDY_STRING_CONCAT_ARGS;
+
 import java.util.Locale;
 
 public class StringTests extends ScriptTestCase {
@@ -70,15 +72,27 @@ public class StringTests extends ScriptTestCase {
     }
     
     public void testAppendMany() {
+        for (int i = MAX_INDY_STRING_CONCAT_ARGS - 5; i < MAX_INDY_STRING_CONCAT_ARGS + 5; i++) {
+            doTestAppendMany(i);
+        }
+    }
+    
+    private void doTestAppendMany(int count) {
         StringBuilder script = new StringBuilder("String s = \"cat\"; return s");
         StringBuilder result = new StringBuilder("cat");
-        for (int i = 0; i < WriterConstants.MAX_INDY_STRING_CONCAT_ARGS + 10; i++) {
-            final String s = String.format(Locale.ROOT,  "%03d", i);
+        for (int i = 1; i < count; i++) {
+            final String s = String.format(Locale.ROOT, "%03d", i);
             script.append(" + '").append(s).append("'.toString()");
             result.append(s);
         }
-        //System.out.println(Debugger.toString(script.toString()));
-        assertEquals(result.toString(), exec(script.toString()));
+        final String s = script.toString();
+        assertTrue("every string part should be separatly pushed to stack.",
+                Debugger.toString(s).contains(String.format(Locale.ROOT, "LDC \"%03d\"", count/2)));
+        assertEquals(result.toString(), exec(s));
+    }
+    
+    public void testNestedConcats() {
+        assertEquals("foo1010foo", exec("String s = 'foo'; String x = '10'; return s + Integer.parseInt(x + x) + s;"));
     }
     
     public void testStringAPI() {