The runtime has encountered a fatal error

Question

hi every buddy,

i got this error 
"
The runtime has encountered a fatal error. The address of the error was at 0x643a9ddd, on thread 0x1674. The error code is 0x80131623. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM-interop or PInvoke, which may corrupt the stack.
"
in my following highlighted code

" TextRange range;
 FileStream fStream;
 range = new TextRange(staticLogTxtBox.Document.ContentStart, staticLogTxtBox.Document.ContentEnd);
fStream = new FileStream(logFilePath, FileMode.Create);
 range.Save(fStream, DataFormats.Text);
fStream.Close();"

so please help me to resolve this problem..
thank you..