1 files changed, 5 insertions, 2 deletions
diff --git a/Lib/tokenize.py b/Lib/tokenize.py
index 741417abd8..c05f764dd8 100644
--- a/Lib/tokenize.py
+++ b/Lib/tokenize.py
@@ -364,9 +364,12 @@ def detect_encoding(readline):
 
     def find_cookie(line):
         try:
-            line_string = line.decode('ascii')
+            # Decode as UTF-8. Either the line is an encoding declaration,
+            # in which case it should be pure ASCII, or it must be UTF-8
+            # per default encoding.
+            line_string = line.decode('utf-8')
         except UnicodeDecodeError:
-            return None
+            raise SyntaxError("invalid or missing encoding declaration")
 
         matches = cookie_re.findall(line_string)
         if not matches: