diff --git a/contrib/mbs-build b/contrib/mbs-build
index 69584f71..bffecf44 100755
--- a/contrib/mbs-build
+++ b/contrib/mbs-build
@@ -249,7 +249,7 @@ def get_scm_branch(branch):
 def submit_module_build(scm_url, branch, server, id_provider, pyrpkg, verify=True, optional=None):
     """
     Submits the module defined by `scm_url` to MBS instance defined
-    by `server`. Returns build_id or negative error code.
+    by `server`. Returns tuple: build_id or negative error code, error message.
     """
     scm_url = get_scm_url(scm_url, pyrpkg)
     branch = get_scm_branch(branch)
@@ -258,7 +258,11 @@ def submit_module_build(scm_url, branch, server, id_provider, pyrpkg, verify=Tru
 
     logging.info("Submitting module build %s", scm_url)
     body = {'scmurl': scm_url, 'branch': branch}
-    body.update({x.split("=")[0]: x.split("=")[1] for x in optional})
+    try:
+        optional_dict = {y[0]: y[1] for y in [x.split("=", 1) for x in optional]}
+    except IndexError:
+        return -5, "Optional arguments are not in a proper arg=value format."
+    body.update(optional_dict)
     resp = send_authorized_request(
         "POST", server, id_provider, "/module-build-service/1/module-builds/",
         body, verify=verify)