diff --git a/src/flask_socketio/test_client.py b/src/flask_socketio/test_client.py
index 312bac1d..f19d7d4b 100644
--- a/src/flask_socketio/test_client.py
+++ b/src/flask_socketio/test_client.py
@@ -128,7 +128,15 @@ def connect(self, namespace=None, query_string=None, headers=None,
             if query_string[0] != '?':
                 query_string = '?' + query_string
             url += query_string
-        environ = EnvironBuilder(url, headers=headers).get_environ()
+
+        environ_base = (
+            None
+            if not self.flask_test_client
+            else self.flask_test_client.environ_base
+        )
+        environ = EnvironBuilder(
+            url, headers=headers, environ_base=environ_base
+        ).get_environ()
         environ['flask.app'] = self.app
         if self.flask_test_client:
             # inject cookies from Flask