diff --git a/zipstream/__init__.py b/zipstream/__init__.py
index 811e2cf..33dc000 100644
--- a/zipstream/__init__.py
+++ b/zipstream/__init__.py
@@ -116,7 +116,7 @@ def FileHeader(self, zip64=None):
         if zip64 is None:
             zip64 = file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT
         if zip64:
-            fmt = '<HHQQ'
+            fmt = b'<HHQQ'
             extra = extra + struct.pack(fmt,
                     1, struct.calcsize(fmt)-4, file_size, compress_size)
         if file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT: