diff --git a/shell.nix b/shell.nix
index 10eb9dfcd..0e93c7272 100644
--- a/shell.nix
+++ b/shell.nix
@@ -157,7 +157,7 @@ let
           [[ -n "$maildev_pid" ]] && kill $maildev_pid
         }
 
-        ${pkgs.nodePackages.maildev}/bin/maildev --smtp $(($PORT_OFFSET + 1025)) --web $(($PORT_OFFSET + 8080)) --ip localhost --web-ip localhost &>/dev/null &
+        TMPDIR=''${XDG_RUNTIME_DIR} ${pkgs.nodePackages.maildev}/bin/maildev --smtp $(($PORT_OFFSET + 1025)) --web $(($PORT_OFFSET + 8080)) --ip localhost --web-ip localhost &>/dev/null &
         maildev_pid=$!
 
         export SMTPHOST=localhost