diff --git a/modules/programs/notmuch.nix b/modules/programs/notmuch.nix
index ef9f2ec4b..939b8ef6d 100644
--- a/modules/programs/notmuch.nix
+++ b/modules/programs/notmuch.nix
@@ -89,7 +89,7 @@ in
       extraConfig = mkOption {
         type = types.attrsOf (types.attrsOf types.str);
         default = {
-          maildir = { synchronize_flags = "True"; };
+          maildir = { synchronize_flags = "true"; };
         };
         description = ''
           Options that should be appended to the notmuch configuration file.