Index: generic/nsfStack.c
===================================================================
diff -u -N -r7c9627dd582ea07050a95a6e0bb05cdf381fe592 -r22741a6333a986af0a38983a0d06580f1a0103e5
--- generic/nsfStack.c	(.../nsfStack.c)	(revision 7c9627dd582ea07050a95a6e0bb05cdf381fe592)
+++ generic/nsfStack.c	(.../nsfStack.c)	(revision 22741a6333a986af0a38983a0d06580f1a0103e5)
@@ -112,9 +112,13 @@
  *
  *----------------------------------------------------------------------
  */
+void NsfShowStack(Tcl_Interp *interp) nonnull(1);
+
 void NsfShowStack(Tcl_Interp *interp) {
   Tcl_CallFrame *framePtr;
 
+  nonnull_assert(interp != NULL);
+
   fprintf(stderr, "NsfShowStack framePtr %p varFramePtr %p\n",
           (void *)Tcl_Interp_framePtr(interp), (void *)Tcl_Interp_varFramePtr(interp));
   /* framePtr = (Tcl_CallFrame *)Tcl_Interp_varFramePtr(interp);