importing issue

Hi and tanx to your grate library 
i have created a file(string.sol) and copy paste the contents into it
the file is in the main solidity directory 
i am importing via import "./strings.sol";
but i get an error for this line     using strings for *;     
error : AttributeError: 'UsingForDirective' object has no attribute 'typeName'
i am using brownie