diff --git a/GNUmakefile b/GNUmakefile
index b6feb1ba..c586c228 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -344,6 +344,7 @@ OO_UTILITY_FILES = \
     OOShipGroup.m \
     OOStringParsing.m \
     OOWeakReference.m \
+    OOWeakSet.m \
     OOXMLExtensions.m \
     OODeepCopy.m \
     OORegExpMatcher.m \