Update readers.py

pandas/io/parsers/readers.py

@@ -1302,7 +1302,7 @@ def _refine_defaults_read(
     if delimiter and (sep is not lib.no_default):
         raise ValueError("Specified a sep and a delimiter; you can only specify one.")
 
-    allowed_names_prefix = {None, lib.no_default}
+    allowed_names_prefix = [None, lib.no_default]
     if names not in allowed_names_prefix and prefix not in allowed_names_prefix:
         raise ValueError("Specified named and prefix; you can only specify one.")