diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 509297003f..409e6256b5 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -517,7 +517,9 @@ static void prim_toPath(EvalState & state, Value * * args, Value & v)
 {
     PathSet context;
     Path path = state.coerceToPath(*args[0], context);
-    mkString(v, canonPath(path), context);
+    if (!context.empty())
+        throw EvalError(format("string `%1%' cannot refer to other paths") % path);
+    mkPath(v, canonPath(path).c_str());
 }