diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index f609e8944..f4478b449 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -15,7 +15,7 @@ namespace nix {
 
 
 #define foreach(it_type, it, collection)                                \
-    for (it_type it = collection.begin(); it != collection.end(); ++it)
+    for (it_type it = (collection).begin(); it != (collection).end(); ++it)
 
 
 /* Return an environment variable. */