Constrain() - warning: second operand of conditional expression has no effect

pert:
The code says 250 but the post says:

right... anyway the correct code that does not generate any warning is

byte a = 20;

void setup() {
  Serial.begin(115200);
  a = constrain(a, 0, 250);  // THIS IS HOW TO DO THAT
  Serial.print("\n\na: ");
  Serial.print(a);
}

void loop() {}