diff --git a/ext/openssl/ossl_digest.c b/ext/openssl/ossl_digest.c
index fc326ec14..015d8eb58 100644
--- a/ext/openssl/ossl_digest.c
+++ b/ext/openssl/ossl_digest.c
@@ -232,6 +232,7 @@ ossl_digest_finish(int argc, VALUE *argv, VALUE self)
         str = rb_str_new(NULL, out_len);
     } else {
         StringValue(str);
+        rb_str_modify(str);
         rb_str_resize(str, out_len);
     }