diff --git a/tests/libqos/malloc-pc.c b/tests/libqos/malloc-pc.c
index 63af60a..be1d97f 100644
--- a/tests/libqos/malloc-pc.c
+++ b/tests/libqos/malloc-pc.c
@@ -36,7 +36,7 @@
 
 
     size += (PAGE_SIZE - 1);
-    size &= PAGE_SIZE;
+    size &= -PAGE_SIZE;
 
     g_assert_cmpint((s->start + size), <=, s->end);