diff --git a/openapi_core/testing/requests.py b/openapi_core/testing/requests.py
index 81e97f24..8590f9ca 100644
--- a/openapi_core/testing/requests.py
+++ b/openapi_core/testing/requests.py
@@ -31,7 +31,7 @@ def __init__(
         self.view_args = view_args
         self.headers = headers
         self.cookies = cookies
-        self.body = data or ""
+        self.body = data or b""
         self.content_type = content_type
 
         self.parameters = RequestParameters(