Android problem: cannot find text file under assets

I put symp.txt under assets and use the following code to load it:

    AssetManager am = context.getAssets();

    InputStream is;

try {

    is = am.open("symp.txt");

    mCategory = new SymptomCategory(is);

} catch (IOException e) {

    e.printStackTrace();

}

However I got exception when opening the file.

The problem was fixed by cleaning up the project and let it auto recompile.