diff --git a/expr.h b/expr.h
index 5abff910..355f34c9 100644
--- a/expr.h
+++ b/expr.h
@@ -65,7 +65,7 @@ struct node_t
   mask_t *    mask;
 
   node_t(const kind_t _type)
-    : type(_type), left(NULL), right(NULL) {}
+    : type(_type), left(NULL), right(NULL), mask(NULL) {}
 
   ~node_t() {
     if (mask)  delete mask;