If the 'KM/H' is not on the sign can it be argued that it is not a legal sign, meaning, I was not given proper warning that the speed was going to drop?

Sign in question - 50 km/h ahead.

The sign had an arrow on top, the number '50' , but there was no 'km/h'. Can it be argued in court that it is not legal, therefore, I did not have adequate time to slow from an 80 zone once I entered a 50 zone?

Thanks