diff --git a/src/xml.coffee b/src/xml.coffee
index 745ad02..3ea0c8c 100644
--- a/src/xml.coffee
+++ b/src/xml.coffee
@@ -126,7 +126,7 @@ class Tag extends EventEmitter
 
     attr: (key, value) =>
         if typeof key is 'string'
-            if not value?
+            if value is undefined
                 attr = @builder?.query('attr', this, key)
                 # attr is not defined if attr is undefined
                 # attr doesn't have a value when attr is null