1 files changed, 6 insertions, 2 deletions
diff --git a/string.c b/string.c
index d10eb236c9..555b0a1abd 100644
--- a/string.c
+++ b/string.c
@@ -3880,8 +3880,10 @@ rb_str_index_m(int argc, VALUE *argv, VALUE str)
     }
 
     if (RB_TYPE_P(sub, T_REGEXP)) {
-        if (pos > str_strlen(str, NULL))
+        if (pos > str_strlen(str, NULL)) {
+            rb_backref_set(Qnil);
             return Qnil;
+        }
         pos = str_offset(RSTRING_PTR(str), RSTRING_END(str), pos,
                          rb_enc_check(str, sub), single_byte_optimizable(str));
 
@@ -3989,8 +3991,10 @@ rb_str_byteindex_m(int argc, VALUE *argv, VALUE str)
     }
 
     if (RB_TYPE_P(sub, T_REGEXP)) {
-        if (pos > RSTRING_LEN(str))
+        if (pos > RSTRING_LEN(str)) {
+            rb_backref_set(Qnil);
             return Qnil;
+        }
         if (rb_reg_search(sub, str, pos, 0) < 0) {
             return Qnil;
         }