diff --git a/fhir-path/src/main/java/com/ibm/fhir/path/function/FHIRPathStringAbstractFunction.java b/fhir-path/src/main/java/com/ibm/fhir/path/function/FHIRPathStringAbstractFunction.java
index cd361b5458e..5386edc7bfe 100644
--- a/fhir-path/src/main/java/com/ibm/fhir/path/function/FHIRPathStringAbstractFunction.java
+++ b/fhir-path/src/main/java/com/ibm/fhir/path/function/FHIRPathStringAbstractFunction.java
@@ -10,6 +10,7 @@
 import static com.ibm.fhir.path.util.FHIRPathUtil.getSingleton;
 import static com.ibm.fhir.path.util.FHIRPathUtil.hasStringValue;
 import static com.ibm.fhir.path.util.FHIRPathUtil.isSingleton;
+import static com.ibm.fhir.path.util.FHIRPathUtil.isStringSubType;
 
 import java.util.Collection;
 import java.util.List;
@@ -40,14 +41,8 @@ public Collection<FHIRPathNode> apply(EvaluationContext evaluationContext, Colle
             
         } else if (!hasStringValue(context)) {
             FHIRPathNode node = getSingleton(context);
-            FHIRPathType nodeChildType = 
-                   node.children() != null && !node.children().isEmpty() 
-                     && node.children().iterator().hasNext() && node.children().iterator().next().type() != null ? node.children().iterator().next().type() : FHIRPathType.FHIR_ANY;
-                    
-            if (nodeChildType == FHIRPathType.FHIR_EXTENSION) {
-                if( logger.isLoggable(Level.FINE) ) {
-                    logger.fine(String.format("collection item is of extension type %s", node.type().getName()));
-                }
+            if (isStringSubType(node)) {
+                logger.fine("collection item does not have a value, returning empty");
                 return empty();
             }
             throw new IllegalArgumentException("Input collection item must be of type String, but found '"+node.type().getName()+ "'");
diff --git a/fhir-path/src/main/java/com/ibm/fhir/path/util/FHIRPathUtil.java b/fhir-path/src/main/java/com/ibm/fhir/path/util/FHIRPathUtil.java
index e9d3f2ed74f..d3c4c9da587 100644
--- a/fhir-path/src/main/java/com/ibm/fhir/path/util/FHIRPathUtil.java
+++ b/fhir-path/src/main/java/com/ibm/fhir/path/util/FHIRPathUtil.java
@@ -1238,4 +1238,21 @@ public static com.ibm.fhir.model.type.String getDisplay(FHIRPathTree tree, FHIRP
         }
         return null;
     }
+    
+    /**
+     * returns true if the node is a String or if FHIR element can be automatically converted to System.String 
+     * https://www.hl7.org/fhir/fhirpath.html#types
+     * @param node
+     * @return boolean
+     */
+    public static boolean isStringSubType(FHIRPathNode node) {
+        return node.type() == FHIRPathType.FHIR_STRING 
+                || node.type() == FHIRPathType.FHIR_URI 
+                || node.type() == FHIRPathType.FHIR_CODE 
+                || node.type() == FHIRPathType.FHIR_OID 
+                || node.type() == FHIRPathType.FHIR_ID  // included for consistency with spec
+                || node.type() == FHIRPathType.FHIR_UUID 
+                || node.type() == FHIRPathType.FHIR_MARKDOWN 
+                || node.type() == FHIRPathType.FHIR_BASE64BINARY;
+    }
 }
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