--- a/src/jdk.internal.vm.compiler/share/classes/org.graalvm.graphio/src/org/graalvm/graphio/ProtocolImpl.java	Fri Mar 16 11:26:05 2018 +0000
+++ b/src/jdk.internal.vm.compiler/share/classes/org.graalvm.graphio/src/org/graalvm/graphio/ProtocolImpl.java	Fri Mar 16 22:59:32 2018 -0700
@@ -128,7 +128,11 @@
 
     @Override
     protected Object findType(Port edges, int i) {
-        return structure.edgeType(edges, i);
+        Object type = structure.edgeType(edges, i);
+        if (findEnumOrdinal(type) < 0) {
+            throw new IllegalStateException("edgeType method shall return an enum! Was: " + type);
+        }
+        return type;
     }
 
     @Override
@@ -138,7 +142,11 @@
 
     @Override
     protected Object findJavaClass(NodeClass clazz) {
-        return structure.nodeClassType(clazz);
+        final Object type = structure.nodeClassType(clazz);
+        if (!(type instanceof Class<?>) && findJavaTypeName(type) == null) {
+            throw new IllegalStateException("nodeClassType method shall return a Java class (instance of Class)! Was: " + type);
+        }
+        return type;
     }
 
     @Override