How to get the correct file in the CSV file input?

Hi,

i have the following data flow:
get file name->csv file input->dummy
my source file is in a share folder e.g. z:\f1.csv
i set the path as a variable ${input} in the get file name step and set the filename field in the csv file input.
when i set the parameter value to z:\f1.csv in my executing , the workflow executed failed:

error:
- ERROR (version 5.1.0.0, build 1 from 2014-06-19_19-02-57 by buildguy) : org.apache.commons.vfs.FileNotFolderException: Could not list the contents of "file:///C:/Program Files/data-integration/${Input}" because it is not a folder.

but when i set the z:\f1.csv as parameter default value , the flow success.

Any one knows the reason?