Workflow BufferTool error

If the Buffer amount in a workflow is 0 (ZERO) feet, then the workflow throws out an exception error that it is "unable to complete the buffer operation".  Is this a Workflow bug?  I can use the buffer tool on the Analysis Toolbar with 0 (ZERO) feet and it works just fine.

I take the user input which is of STRING type and convert it to System.Double type using the A=B Assign tool:   buffer = Cdbl(bufferstr)