diff --git a/src/trap.c b/src/trap.c
index a5602cd5a..41f2cad94 100644
--- a/src/trap.c
+++ b/src/trap.c
@@ -6400,7 +6400,7 @@ chest_trap(
         bot(); /* to get immediate botl re-display */
     }
 
-    otmp->tknown = 1; /* hero knows chest is no longer trapped */
+    obj->tknown = 1; /* hero knows chest is no longer trapped */
     return FALSE;
 }