diff --git a/gunicorn/glogging.py b/gunicorn/glogging.py
index e614294d..60745f98 100644
--- a/gunicorn/glogging.py
+++ b/gunicorn/glogging.py
@@ -469,8 +469,7 @@ class Logger(object):
                     # so we need to convert it to a byte string
                     auth = base64.b64decode(auth[1].strip().encode('utf-8'))
                     # b64decode returns a byte string
-                    auth = auth.decode('utf-8')
-                    auth = auth.split(":", 1)
+                    auth.split(b":", 1)[0].decode(UTF-8", "replace")
                 except (TypeError, binascii.Error, UnicodeDecodeError) as exc:
                     self.debug("Couldn't get username: %s", exc)
                     return user