How can I get the internal "message" value of a System.Exception?

I have an FTP exception thrown from a third-party assembly which is quite generic:

Exception of type 'JSchToCSharp.SharpSsh.jsch.SftpException' was thrown.

On inspection of the Exception, I see there is a private/internal member called message (lower-case m) that contains my error message:

How can I get the value of this message member?

I have tried to use reflection to get it but null is returned from GetValue:

    BindingFlags bindFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic
        | BindingFlags.Static;
    FieldInfo field = type.GetField(fieldName, bindFlags);
    var value = field.GetValue(instance);

    return value.ToString();

It doesn't appear to be Non-Public or Static so I'm a little unsure as to what to use as my BindingFlags.

Thanks