diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index d18fffa..d8ccb44 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -963,10 +963,14 @@ struct tok_state *
         if (tok->lineno > 2) {
             tok->decoding_state = STATE_NORMAL;
         }
-        else if (!check_coding_spec(tok->cur, tok->end - tok->cur,
-                                    tok, fp_setreadl))
-        {
-            return 0;
+        else {
+            Py_ssize_t size = tok->end - tok->cur;
+            Py_ssize_t len = strnlen(tok->cur, size);
+            size = Py_MIN(size, len);
+            if (!check_coding_spec(tok->cur, size, tok, fp_setreadl))
+            {
+                return 0;
+            }
         }
     }
     /* The default encoding is UTF-8, so make sure we don't have any