diff --git a/sphinx/environment.py b/sphinx/environment.py
index 0884eccad..522e2bd06 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -534,7 +534,7 @@ class BuildEnvironment:
         """
         Custom decoding error handler that warns and replaces.
         """
-        linestart = error.object.rfind('\n', None, error.start)
+        linestart = error.object.rfind('\n', 0, error.start)
         lineend = error.object.find('\n', error.start)
         if lineend == -1: lineend = len(error.object)
         lineno = error.object.count('\n', 0, error.start) + 1