[Jack Jones]

As I said in your other post, the negative of an external gear does not result in the proper profile of an internal gear.

The profile of an internal gear may also need to be undercut:

UNDERCUT
When the number of teeth in a gear is small, the tip of the mating
gear tooth may interfere with the lower portion of the tooth profile.
To prevent this, the generating process removes material at
this point. This results in loss of a portion of the involute adjacent
to the tooth base, reducing tooth contact and tooth strength.
On 14-1/2°PA gears undercutting occurs where a number of
teeth is less than 32 and for 20°PA less than 18. Since this
condition becomes more severe as tooth numbers decrease, it
is recommended that the minimum number of teeth be 16 for
14-1/2°PA and 13 for 20°PA.

In a similar manner INTERNAL Spur Gear teeth may interfere
when the pinion gear is too near the size of its mating internal
gear. The following may be used as a guide to assure proper
operation of the gear set. For 14-1/2°PA, the difference in
tooth numbers between the gear and pinion should not be less
than 15. For 20°PA the difference in tooth numbers should not
be less than 12.