.net - What To Do When Dll Return A String Not Correctly Terminated

I have an third part dll that have a function that returns a string.When I call the function I got for example "123456" back. At least it seams like that, but when i do mystring.length it does not return any length. If I set the text property of a label it shows "123456".

When I have the string I got from the dll I send it to a webservice function:

rem call dll and get string
dim mystring as string=mydll.getstring()
rem Send it to the webservice
webservice.SaveString(mystring)

The webservice crashes with an webexeption ("protocoll error") when I send in the string I got from the dll. If I instead send it like this:

rem call dll and get string
dim mystring as string=mydll.getstring()
rem Send it to the webservice
dim FixedString as string = mid(mystring,1,6)
webservice.SaveString(mystring)

Then it works and everything is fine. That leads me to think that the string I got from the DLL is in someway not terminated correct.And I dont know how to fix it from vb.net (I cant change the DLL)and I dont know in beforehand how long the string will be.The dll does only work on site when specific hardware is connected so I can't sit at the office trying to fix this in dev-environment. So I would like to have some possibly solutions to this before I go to the customer again.

Edit 1:I tried to just do a loop that looped from 1 to 100 (because I dont know the real length) and tried to copy all characters that <>"" to a new variable but it didnt work. I didnt try other variants because the day was over and I had to leave the customer site.

Edit 2:The DLL Im talking to is made in VB6. That DLL is talking to other DLLs that is made in C++ (I think).

My program < - > vb6-DLL <-> c++-DLL <-> hardware