--- a/graal/com.oracle.truffle.api/src/com/oracle/truffle/api/instrument/Instrument.java	Tue May 12 18:04:53 2015 -0700
+++ b/graal/com.oracle.truffle.api/src/com/oracle/truffle/api/instrument/Instrument.java	Tue May 12 18:26:52 2015 -0700
@@ -466,7 +466,10 @@
                 if (requiredResultType == null) {
                     return;
                 }
-                if (result == null || !(requiredResultType.isAssignableFrom(result.getClass()))) {
+                if (result == null) {
+                    throw new RuntimeException("Instrument result null: " + requiredResultType.getSimpleName() + " is required");
+                }
+                if (!(requiredResultType.isAssignableFrom(result.getClass()))) {
                     throw new RuntimeException("Instrument result " + result.toString() + " not assignable to " + requiredResultType.getSimpleName());
                 }
             }