1 files changed, 1 insertions, 0 deletions
diff --git a/string.c b/string.c
index d70ea21868..75e828e7d0 100644
--- a/string.c
+++ b/string.c
@@ -1408,6 +1408,7 @@ rb_str_upto(beg, end, excl)
 	rb_yield(current);
 	if (!excl && rb_str_equal(current, end)) break;
 	current = rb_funcall(current, succ, 0, 0);
+	StringValue(current);
 	if (excl && rb_str_equal(current, end)) break;
 	if (RSTRING(current)->len > RSTRING(end)->len)
 	    break;