1 files changed, 1 insertions, 1 deletions
diff --git a/array.c b/array.c
index 19d9946a01..3aab2ec2a3 100644
--- a/array.c
+++ b/array.c
@@ -5209,7 +5209,7 @@ flatten(VALUE ary, int level)
 	    }
 	    else {
 		id = (st_data_t)tmp;
-		if (st_lookup(memo, id, 0)) {
+		if (st_is_member(memo, id)) {
                     st_clear(memo);
 		    rb_raise(rb_eArgError, "tried to flatten recursive array");
 		}