diff --git a/sphinxapi/__init__.py b/sphinxapi/__init__.py
index 3be9023..602c841 100644
--- a/sphinxapi/__init__.py
+++ b/sphinxapi/__init__.py
@@ -602,7 +602,7 @@ class SphinxClient:
         # filters
         req.append(pack('>L', len(self._filters)))
         for f in self._filters:
-            req.append(pack('>L', len(f['attr'])) + f['attr'])
+            req.append(pack('>L', len(f['attr'])) + f['attr'].encode())
             filtertype = f['type']
             req.append(pack('>L', filtertype))
             if filtertype == SPH_FILTER_VALUES: