diff --git a/modules/programs/astroid.nix b/modules/programs/astroid.nix
index 45c35fa9..0463cd15 100644
--- a/modules/programs/astroid.nix
+++ b/modules/programs/astroid.nix
@@ -106,6 +106,14 @@ in
   };
 
   config = mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = config.programs.notmuch.maildir.synchronizeFlags;
+        message = "The astroid module requires"
+          + " 'programs.notmuch.maildir.synchronizeFlags = true'.";
+      }
+    ];
+
     home.packages =  [ pkgs.astroid ];
 
     xdg.configFile."astroid/config".source =