diff --git a/src/tests/client_server/libtest.h b/src/tests/client_server/libtest.h
index f1ac5e8c..18c4634a 100644
--- a/src/tests/client_server/libtest.h
+++ b/src/tests/client_server/libtest.h
@@ -280,7 +280,7 @@ struct MHDT_Phase
   /**
    * Closure for @e server_cb.
    */
-  const void *server_cb_cls;
+  void *server_cb_cls;
 
   /**
    * Logic for the CURL client for this phase.