Refined assertion.

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/function/GetCallerFrameNode.java

@@ -61,9 +61,8 @@ public final class GetCallerFrameNode extends RBaseNode {
             if (slowPathFrame != null) {
                 return slowPathFrame;
             }
-            callerFrameObject = null;
         }
-        assert callerFrameObject == null;
+        assert callerFrameObject instanceof CallerFrameClosure || callerFrameObject == null;
 
         // S3 method can be dispatched from top-level where there is no caller frame
         // Since RArguments does not allow to create arguments with a 'null' caller frame, this