diff --git a/pkgs/desktops/pantheon/update.nix b/pkgs/desktops/pantheon/update.nix
index 50f1fbd48460..7e9aacc0c2b2 100644
--- a/pkgs/desktops/pantheon/update.nix
+++ b/pkgs/desktops/pantheon/update.nix
@@ -29,6 +29,8 @@ let
     chmod +x $out
   '';
 
-  versionFlag = { "release" = "-r"; "master" = "-m"; }.${versionPolicy};
+  throwFlag = throw "${versionPolicy} is not a valid versionPolicy - Options are either 'release' or 'master' (defaults to release).";
+
+  versionFlag = { "release" = "-r"; "master" = "-m"; }.${versionPolicy} or throwFlag;
 
 in [ updateScript versionFlag repoName attrPath ]