diff --git a/pkgs/lib/types.nix b/pkgs/lib/types.nix
index 2c85ee0077a4..956368138aa2 100644
--- a/pkgs/lib/types.nix
+++ b/pkgs/lib/types.nix
@@ -89,7 +89,7 @@ rec {
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.
-      check = lib.traceValIfNot (x: builtins.substring 0 1 (toString x) == "/");
+      check = lib.traceValIfNot (x: builtins.unsafeDiscardStringContext (builtins.substring 0 1 (toString x)) == "/");
     };
 
     listOf = types.list;