--- a/jaxp/src/java.xml/share/classes/com/sun/org/apache/xpath/internal/compiler/FunctionTable.java	Wed Jul 05 20:39:43 2017 +0200
+++ b/jaxp/src/java.xml/share/classes/com/sun/org/apache/xpath/internal/compiler/FunctionTable.java	Tue Jun 30 12:04:27 2015 +0200
@@ -378,6 +378,12 @@
     int funcIndex;
     Object funcIndexObj = getFunctionID(name);
 
+    if (func != null && !Function.class.isAssignableFrom(func)) {
+        throw new ClassCastException(func.getName()
+                  + " cannot be cast to "
+                  + Function.class.getName());
+    }
+
     if (null != funcIndexObj)
     {
       funcIndex = ((Integer) funcIndexObj).intValue();