diff --git a/master.cfg b/master.cfg
index d53de5f..d0acb97 100644
--- a/master.cfg
+++ b/master.cfg
@@ -75,7 +75,8 @@ def schedule_force_build(name):
                 branch=forcesched.StringParameter(
                     name="branch",
                     label="Branch:",
-                    default="master"),
+                    default="master",
+                    regex=r'^[a-zA-Z0-9][A-Za-z0-9_-]*$'),
                 # Hide revision. We don't want to allow anyone to overwrite the
                 # master build with an older version. Could be added back once we
                 # have authentication.