diff --git a/src/hpack/hpack.py b/src/hpack/hpack.py
index cc39bfd..612bfbd 100644
--- a/src/hpack/hpack.py
+++ b/src/hpack/hpack.py
@@ -143,14 +143,16 @@ def _dict_to_iterable(header_dict):
         yield key, header_dict[key]
 
 
-def _to_bytes(string):
+def _to_bytes(value):
     """
-    Convert string to bytes.
+    Convert anything to bytes through a UTF-8 encoded string
     """
-    if not isinstance(string, basestring):  # pragma: no cover
-        string = str(string)
-
-    return string if isinstance(string, bytes) else string.encode('utf-8')
+    t = type(value)
+    if t is bytes:
+        return value
+    if t is not str:
+        value = str(value)
+    return value.encode("utf-8")
 
 
 class Encoder: