diff --git a/cco/common/report.py b/cco/common/report.py
index 7b6d965..a9fc73d 100755
--- a/cco/common/report.py
+++ b/cco/common/report.py
@@ -38,7 +38,7 @@ class RelationField(BaseRelationField):
         return self.getRawValue(row)
 
     def getSortValue(self, row):
-        return self.getValue(row)
+        return self.getValue(row) or ''
 
     def getExportValue(self, row, format=''):
         return self.getValue(row)