diff --git a/src/libexpr/tests/libexprtests.hh b/src/libexpr/tests/libexprtests.hh
index 418da6b6d..0286be3b3 100644
--- a/src/libexpr/tests/libexprtests.hh
+++ b/src/libexpr/tests/libexprtests.hh
@@ -23,7 +23,7 @@ namespace nix {
             }
             Value eval(std::string input, bool forceValue = true) {
                 Value v;
-                Expr * e = state.parseExprFromString(input, "");
+                Expr * e = state.parseExprFromString(input, state.rootPath("/"));
                 assert(e);
                 state.eval(e, v);
                 if (forceValue)