diff --git a/includes/migration.inc b/includes/migration.inc
index fbfad66..22e01f1 100644
--- a/includes/migration.inc
+++ b/includes/migration.inc
@@ -810,7 +810,8 @@ abstract class Migration extends MigrationBase {
 
         // When updating existing items, make sure we don't create a destination
         // field that is not mapped to anything (a source field or a default value)
-        if (!$source && !$default) {
+        // although zero may be a valid value
+        if (!$source && $default !== 0 && !$default) {
           continue;
         }