diff --git a/pkgs/applications/emulators/np2kai/default.nix b/pkgs/applications/emulators/np2kai/default.nix
index cf55085b78a1..4e2f01ade369 100644
--- a/pkgs/applications/emulators/np2kai/default.nix
+++ b/pkgs/applications/emulators/np2kai/default.nix
@@ -124,7 +124,7 @@ stdenv.mkDerivation rec {
 
   configurePhase = ''
     export GIT_VERSION=${builtins.substring 0 7 src.rev}
-    buildFlags="$buildFlags ''${enableParallelBuilding:+-j$NIX_BUILD_CORES"
+    buildFlags="$buildFlags ''${enableParallelBuilding:+-j$NIX_BUILD_CORES}"
   '' + optionalString enableX11 ''
     cd x11
     substituteInPlace Makefile.am \