AvroStorage can't read multiple files?

[Yang]

I had this code:

a = load 'myfile.avro' USING AvrotStorage();
....

it works fine.

but after I change 'myfile' to 'mydirectory/path/' or 'mydirectory/path/*',
it loads nothing, and gave an error :"Schema for a unknown."

I guess the AvroStorage() does not provide the ability to load multiple
files? ----- I remember the default PigStorage does allow file globs to be
loaded

thanks!
yang