diff --git a/string.c b/string.c
index 2ccb0a8ab4..5b556679c7 100644
--- a/string.c
+++ b/string.c
@@ -1025,7 +1025,7 @@ str_new_frozen(VALUE klass, VALUE orig)
 	str = str_new(klass, RSTRING_PTR(orig), RSTRING_LEN(orig));
     }
     else {
-	if (FL_TEST(orig, STR_SHARED)) {
+	if (FL_TEST_RAW(orig, STR_SHARED)) {
 	    VALUE shared = RSTRING(orig)->as.heap.aux.shared;
 	    long ofs = RSTRING(orig)->as.heap.ptr - RSTRING(shared)->as.heap.ptr;
 	    long rest = RSTRING(shared)->as.heap.len - ofs - RSTRING(orig)->as.heap.len;