invalid server's version String Tamirsharpssh

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Java

while connecting from one particular server, I am getting this error. While in other server it's running fine.

Following code causes problem:

if(i==buf.buffer.Length ||
i<7 ||                                      // SSH-1.99 or SSH-2.0
(buf.buffer[4]=='1' && buf.buffer[6]!='9')  // SSH-1.5
)
{
throw new JSchException("invalid server's version String ");
}

The below condition becomes true:

i==buf.buffer.Length

Any idea why it's failing on one particular server?