diff --git a/pkgs/build-support/build-fhs-userenv-bubblewrap/default.nix b/pkgs/build-support/build-fhs-userenv-bubblewrap/default.nix
index d62e4d2a96f8..4604b92313f5 100644
--- a/pkgs/build-support/build-fhs-userenv-bubblewrap/default.nix
+++ b/pkgs/build-support/build-fhs-userenv-bubblewrap/default.nix
@@ -194,7 +194,7 @@ let
 
   bin = writeShellScriptBin name (bwrapCmd { initArgs = ''"$@"''; });
 
-  versionStr = lib.optionalString (! isNull version) ("-" + version);
+  versionStr = lib.optionalString (version != null) ("-" + version);
 
   nameAndVersion = name + versionStr;
 
diff --git a/pkgs/build-support/build-fhs-userenv/default.nix b/pkgs/build-support/build-fhs-userenv/default.nix
index ab890387e6d2..6f0adfb4e08c 100644
--- a/pkgs/build-support/build-fhs-userenv/default.nix
+++ b/pkgs/build-support/build-fhs-userenv/default.nix
@@ -23,7 +23,7 @@ let
     exec ${run} "$@"
   '';
 
-  versionStr = lib.optionalString (! isNull version) ("-" + version);
+  versionStr = lib.optionalString (version != null) ("-" + version);
 
   nameAndVersion = name + versionStr;