diff --git a/preproc.c b/preproc.c
index fcec93d6..bbe2dae9 100644
--- a/preproc.c
+++ b/preproc.c
@@ -4804,6 +4804,7 @@ static void pp_cleanup(int pass)
         }
 
         free_mmacro(defining);
+        defining = NULL;
     }
     while (cstk)
         ctx_pop();