Index: generic/nsf.c
===================================================================
diff -u -r94a8ea7c8b50082b0b6e34e4713669dd77caa7b5 -r5368627bc1d475cf87ed629f6c70ee78654a4db6
--- generic/nsf.c	(.../nsf.c)	(revision 94a8ea7c8b50082b0b6e34e4713669dd77caa7b5)
+++ generic/nsf.c	(.../nsf.c)	(revision 5368627bc1d475cf87ed629f6c70ee78654a4db6)
@@ -20560,7 +20560,11 @@
 
       for (;;) {
         varFramePtr = Tcl_CallFrame_callerPtr(varFramePtr);
-        if (((unsigned int)Tcl_CallFrame_isProcCallFrame(varFramePtr) & (FRAME_IS_NSF_METHOD|FRAME_IS_NSF_CMETHOD)) == 0) {
+        if (unlikely(varFramePtr == NULL)) {
+          break;
+        }
+        if (((unsigned int)Tcl_CallFrame_isProcCallFrame(varFramePtr)
+             & (FRAME_IS_NSF_METHOD|FRAME_IS_NSF_CMETHOD)) == 0) {
           /*
            * Parent frame is not an NSF frame.
            */