diff --git a/suitcase/structure.py b/suitcase/structure.py
index 0ea743b..ded3265 100644
--- a/suitcase/structure.py
+++ b/suitcase/structure.py
@@ -56,8 +56,9 @@ def write(self, stream):
             data = stream.getvalue()
             for field, offset in crc_fields:
                 stream.seek(offset)
-                checksum_data = self.crc_field.packed_checksum(data)
+                checksum_data = field.packed_checksum(data)
                 stream.write(checksum_data)
+                data = stream.getvalue()
 
     def unpack(self, data, trailing=False):
         stream = BytesIO(data)