diff --git a/edosl0util/rdrgen.py b/edosl0util/rdrgen.py
index 1275472bf5284cdc9e56ac90c9237a0a4ddeac62..dea4500b4b8395763553729450680fd04efe1122 100644
--- a/edosl0util/rdrgen.py
+++ b/edosl0util/rdrgen.py
@@ -135,9 +135,9 @@ class BinnedTemporaryFileManager(object):
         if not file_obj:
             file_path = self._file_paths.get(bin_key)
             if file_path:
-                file_obj = open(file_path, 'a+')
+                file_obj = open(file_path, 'a+b')
             else:
-                file_obj = tempfile.NamedTemporaryFile(dir=self.dir, delete=False)
+                file_obj = tempfile.NamedTemporaryFile(dir=self.dir, delete=False, mode='wb')
                 file_path = file_obj.name
                 self._file_paths[bin_key] = file_path
             if len(self._file_objs) == self.max_open_files: