diff --git a/google/cloud/storage/_signing.py b/google/cloud/storage/_signing.py
index 1382ebc77..f2e94ba7b 100644
--- a/google/cloud/storage/_signing.py
+++ b/google/cloud/storage/_signing.py
@@ -77,7 +77,7 @@ def get_signed_query_params_v2(credentials, expiration, string_to_sign):
               signed payload.
     """
     ensure_signed_credentials(credentials)
-    signature_bytes = credentials.sign_bytes(string_to_sign)
+    signature_bytes = credentials.sign_bytes(string_to_sign.encode("ascii"))
     signature = base64.b64encode(signature_bytes)
     service_account_name = credentials.signer_email
     return {
diff --git a/tests/unit/test__signing.py b/tests/unit/test__signing.py
index b69f550c1..7cf817cbe 100644
--- a/tests/unit/test__signing.py
+++ b/tests/unit/test__signing.py
@@ -255,7 +255,7 @@ def test_it(self):
             "Signature": base64.b64encode(sig_bytes),
         }
         self.assertEqual(result, expected)
-        credentials.sign_bytes.assert_called_once_with(string_to_sign)
+        credentials.sign_bytes.assert_called_once_with(string_to_sign.encode("ascii"))
 
 
 class Test_get_canonical_headers(unittest.TestCase):
@@ -420,7 +420,7 @@ def _generate_helper(
 
         string_to_sign = "\n".join(elements)
 
-        credentials.sign_bytes.assert_called_once_with(string_to_sign)
+        credentials.sign_bytes.assert_called_once_with(string_to_sign.encode("ascii"))
 
         scheme, netloc, path, qs, frag = urllib_parse.urlsplit(url)
         expected_scheme, expected_netloc, _, _, _ = urllib_parse.urlsplit(