diff --git a/string.c b/string.c
index 31c3f11045..4554699861 100644
--- a/string.c
+++ b/string.c
@@ -3979,6 +3979,20 @@ rb_str_index_m(int argc, VALUE *argv, VALUE str)
     return LONG2NUM(pos);
 }

+/* whether given pos is valid character boundary or not
+ * Note that in this function, "character" means a code point
+ * (Unicode scalar value), not a grapheme cluster.
+ */
+bool
+str_check_byte_pos(VALUE str, long pos)
+{
+    const char *s = RSTRING_PTR(str);
+    const char *e = RSTRING_END(str);
+    const char *p = s + pos;
+    const char *pp = rb_enc_left_char_head(s, p, e, rb_enc_get(str));
+    return p == pp;
+}
+
 /*
  *  call-seq:
  *    byteindex(substring, offset = 0) -> integer or nil
@@ -4040,6 +4054,10 @@ rb_str_byteindex_m(int argc, VALUE *argv, VALUE str)
         }
     }

+    if (!str_check_byte_pos(str, pos)) {
+        rb_raise(rb_eArgError, "invalid pos");
+    }
+
     if (RB_TYPE_P(sub, T_REGEXP)) {
         if (pos > RSTRING_LEN(str))
             return Qnil;