python - 'NoneType' object has no attribute

I am new to python geospatial programming. i ran the following script and got the corresponding error message

>>> import osgeo
>>> import osgeo.ogr
>>> shapefile = osgeo.ogr.Open("tl_2009_us_state.shp")
>>> numLayers = shapefile.GetLayerCount()

Traceback (most recent call last):   

    File "", line 1, in 
    numLayers = shapefile.GetLayerCount() AttributeError: 'NoneType' object has no attribute 'GetLayerCount'