diff -u a/domain_source/domain_source.module b/domain_source/domain_source.module
--- a/domain_source/domain_source.module
+++ b/domain_source/domain_source.module
@@ -174,7 +174,7 @@ function domain_source_node_validate($node, $form) {
   if (!empty($node->domains_raw)) {
     $allowed = array_merge($allowed, $node->domains_raw);
   }
-  if ($node->domain_site) {
+  if (isset($node->domain_site)) {
     // Any assignment is acceptable if sent to all affiliates.
     // I find this code easier to read than a compound IF statement.
   }