diff --git a/nixos/modules/services/misc/calibre-server.nix b/nixos/modules/services/misc/calibre-server.nix
index b8648770ff4..a920aa22ccd 100644
--- a/nixos/modules/services/misc/calibre-server.nix
+++ b/nixos/modules/services/misc/calibre-server.nix
@@ -22,6 +22,7 @@ in
         description = ''
           The directory where the Calibre library to serve is.
           '';
+          type = types.path;
       };
 
     };