diff --git a/atreal_openads/utils.py b/atreal_openads/utils.py
index ccd6828..59a10b3 100644
--- a/atreal_openads/utils.py
+++ b/atreal_openads/utils.py
@@ -110,7 +110,7 @@ def get_file_digest(content, chunk_size=100000):
         """Read 'chunk_size' amount of data from the content."""
         return content.read(chunk_size)
 
-    for chunk in iter(read_chunk, ''):
+    for chunk in iter(read_chunk, b''):
         digest.update(chunk)
     return digest.hexdigest()