diff --git a/zipstream.py b/zipstream.py
index a285897..c0020be 100644
--- a/zipstream.py
+++ b/zipstream.py
@@ -305,7 +305,7 @@ def zip_file(self, filename, arcname=None, compress_type=None):
             zinfo.compress_size = compress_size
         else:
             zinfo.compress_size = file_size
-        zinfo.CRC = CRC
+        zinfo.CRC = abs(CRC)
         zinfo.file_size = file_size
         yield self.update_data_ptr(zinfo.DataDescriptor())
         self.filelist.append(zinfo)