Serial.readbytesuntil()

[Bhuvanesh123]

void setup()
{
  Serial.begin(9600);
}
void loop()
{char a[10];

  if(Serial.available())
  {
    byte b=Serial.readBytesUntil(b,a,10  );
    Serial.println(b);
  }
}

this is an arduino code

here terminator string is b
input:adb
actuall output:3

but expected out put is 2.how could it include terminator character as it made to terminate..help me