Throw better exception on wrong dynamic_templates syntax

server/src/main/java/org/elasticsearch/index/mapper/RootObjectMapper.java

@@ -168,6 +168,9 @@ protected boolean processField(RootObjectMapper.Builder builder, String fieldNam
                       }
                   ]
                 */
+                if ((fieldNode instanceof List) == false) {
+                    throw new MapperParsingException("Dynamic template syntax error, expects an array of named objects");
+                }
                 List<?> tmplNodes = (List<?>) fieldNode;
                 List<DynamicTemplate> templates = new ArrayList<>();
                 for (Object tmplNode : tmplNodes) {

server/src/test/java/org/elasticsearch/index/mapper/RootObjectMapperTests.java

@@ -185,4 +185,21 @@ public void testIllegalFormatField() throws Exception {
             assertEquals("Invalid format: [[test_format]]: expected string value", e.getMessage());
         }
     }
+
+    public void testIllegalDynamicTemplates() throws Exception {
+        String mapping = Strings.toString(XContentFactory.jsonBuilder()
+                .startObject()
+                .startObject("type")
+                    .startObject("dynamic_templates")
+                    .endObject()
+                .endObject()
+            .endObject());
+
+        DocumentMapperParser parser = createIndex("test").mapperService().documentMapperParser();
+        MapperParsingException e = expectThrows(MapperParsingException.class,
+                    () -> parser.parse("type", new CompressedXContent(mapping)));
+            assertEquals("Dynamic template syntax error, expects an array of named objects", e.getMessage());
+    }
+}
+
 }