diff --git a/src/_zkapauthorizer/_storage_server.py b/src/_zkapauthorizer/_storage_server.py
index e7d16a7db548d388967021b301127117368fb92e..d7f4835f65d31267cadc4a7cf5125f4a4401d3bc 100644
--- a/src/_zkapauthorizer/_storage_server.py
+++ b/src/_zkapauthorizer/_storage_server.py
@@ -515,10 +515,10 @@ def get_storage_index_share_size(sharepath):
     with open(sharepath) as share_file:
         header = share_file.read(calcsize(fmt))
 
-    if len(share_data_length_bytes) != calcsize(fmt):
+    if len(header) != calcsize(fmt):
         raise ValueError(
             "Tried to read 4 bytes of share data length from share, got {!r} instead.".format(
-                share_data_length_bytes,
+                header,
             ),
         )