diff --git a/nptdms/base_segment.py b/nptdms/base_segment.py
index 3f40642..724f6a7 100644
--- a/nptdms/base_segment.py
+++ b/nptdms/base_segment.py
@@ -21,6 +21,7 @@ def __init__(self, path):
         self.data_size = 0
         self.has_data = False
         self.data_type = None
+        self.unused = None
 
     def read_raw_data_index(self, file, raw_data_index_header, endianness):
         """ Read the raw data index for a single object in a segment