diff --git a/includes/base.inc b/includes/base.inc
index 383f59a..6acb895 100644
--- a/includes/base.inc
+++ b/includes/base.inc
@@ -804,7 +804,7 @@ abstract class MigrationBase {
             default:
               // CAST(highwater AS INTEGER) would be ideal, but won't
               // work in MySQL. This hack is thought to be portable.
-              $query->where('(highwater+0) < :highwater', array(':highwater' => $highwater));
+              $query->where('(IF(highwater IS NULL OR highwater = \'\', 0, highwater+0)) < :highwater', array(':highwater' => $highwater));
           }
         }
         else {