diff --git a/src/lib/Hydra/Helper/Nix.pm b/src/lib/Hydra/Helper/Nix.pm
index a8ba1ff3..9ecb4d7a 100644
--- a/src/lib/Hydra/Helper/Nix.pm
+++ b/src/lib/Hydra/Helper/Nix.pm
@@ -396,7 +396,7 @@ sub pathIsInsidePrefix {
 
         # ‘..’ should not take us outside of the prefix.
         if ($c eq "..") {
-            return if length($cur) <= length($prefix);
+            return undef if length($cur) <= length($prefix);
             $cur =~ s/\/[^\/]*$// or die; # remove last component
             next;
         }