Matt's solution is correct for removing null characters from a string (+5 to Matt)
There is something else you may consider here, I see that you are using ASCII enoding which is limited to 128 characters. You may want to check that you are using the correct encoding for your data stream. I couldn't tell the amount of hair pulling and calling upon deities that I have seen from people when attempting to debug their application only to find they were using the wrong encoding scheme.
This remark is from MS:
"The ASCIIEncoding object that is returned by this property might not have the appropriate behavior for your application"
Source:
Encoding.ASCII Property [
^]
Good luck.