diff --git a/ninja/testing/client.py b/ninja/testing/client.py
index 940fc695b..0b8a41880 100644
--- a/ninja/testing/client.py
+++ b/ninja/testing/client.py
@@ -133,7 +133,7 @@ def _build_request(
         request = Mock(spec=HttpRequest)
         request.method = method
         request.path = path
-        request.body = ""
+        request.body = b""
         request.COOKIES = {}
         request._dont_enforce_csrf_checks = True
         request.is_secure.return_value = False