diff --git a/pkgs/tools/text/groff/default.nix b/pkgs/tools/text/groff/default.nix
index 4e2f6af0669..fc6b1abf24d 100644
--- a/pkgs/tools/text/groff/default.nix
+++ b/pkgs/tools/text/groff/default.nix
@@ -57,8 +57,8 @@ stdenv.mkDerivation rec {
   # Remove example output with (random?) colors and creation date
   # to avoid non-determinism in the output.
   postInstall = ''
-    rm $doc/share/doc/groff/examples/hdtbl/*color*ps
-    find $doc/share/doc/groff/ -type f -print0 | xargs -0 sed -i -e 's/%%CreationDate: .*//'
+    rm "$doc"/share/doc/groff/examples/hdtbl/*color*ps
+    find "$doc"/share/doc/groff/ -type f -print0 | xargs -0 sed -i -e 's/%%CreationDate: .*//'
     for f in 'man.local' 'mdoc.local'; do
         cat '${./site.tmac}' >>"$out/share/groff/site-tmac/$f"
     done