diff --git a/src/expand.c b/src/expand.c
index 91a4e181..24cdaac0 100644
--- a/src/expand.c
+++ b/src/expand.c
@@ -142,8 +142,8 @@ expand(pic_state *pic, pic_value obj, struct syntactic_env *env)
   case PIC_TT_PORT:
   case PIC_TT_ENV:
   case PIC_TT_UNDEF:
-    pic_error(pic, "logic flaw");
-    abort();			/* unreachable */
+    pic_error(pic, "unexpected value type");
+    return pic_undef_value();	/* unreachable */
   }
 }