diff --git a/pkgs/build-support/docker/default.nix b/pkgs/build-support/docker/default.nix
index 11d5c2b3642..7ff325382a6 100644
--- a/pkgs/build-support/docker/default.nix
+++ b/pkgs/build-support/docker/default.nix
@@ -333,7 +333,7 @@ rec {
 
       chmod ug+w layer
 
-      if [[ -n $extraCommands ]]; then
+      if [[ -n "$extraCommands" ]]; then
         (cd layer; eval "$extraCommands")
       fi