Arabic Grammar Verb Form III

[yuosef46]

In the Arabic Grammar of verb form III I noticed that the verbal noun contain an extra character (ا)
My code is just as simple as this

MyVerb = v3 "كتب";

Then get its output.
The original output of the verbal noun is: مُكَاتَبَاَة
Original output without tones: مكاتباة.

But as a native speaker I think the correct output should be something like this: مُكَاتَبَة
Without tones: مكاتبة