diff --git a/module_build_service/builder/MockModuleBuilder.py b/module_build_service/builder/MockModuleBuilder.py
index 5fb4cabd..df9a1b86 100644
--- a/module_build_service/builder/MockModuleBuilder.py
+++ b/module_build_service/builder/MockModuleBuilder.py
@@ -413,8 +413,8 @@ class MockModuleBuilder(GenericBuilder):
         # ...and inject modules.yaml there if asked.
         if include_module_yaml:
             mmd_path = os.path.join(path, "modules.yaml")
-            with open(mmd_path, "w") as f:
-                f.write(mmd_to_str(m1_mmd))
+            with open(mmd_path, "wb") as f:
+                f.write(mmd_to_str(m1_mmd).encode("utf-8"))
             execute_cmd(["/usr/bin/modifyrepo_c", "--mdtype=modules", mmd_path, repodata_path])
 
     def _add_repo(self, name, baseurl, extra=""):