diff --git a/src/lj_carith.c b/src/lj_carith.c
index 53257e18..f675bf5e 100644
--- a/src/lj_carith.c
+++ b/src/lj_carith.c
@@ -81,7 +81,7 @@ static int carith_checkarg(lua_State *L, CTState *cts, CDArith *ca)
       }
     } else {
       ca->ct[i] = NULL;
-      ca->p[i] = NULL;
+      ca->p[i] = (void *)(intptr_t)1;  /* To make it unequal. */
       ok = 0;
     }
   }