Description of Bug: Yellow sigils seem to be unable to be extended. I tested it on a 30 charge yellow with a 25 and a 18 charge crystal. After about 10-15 effective casts, not a single pip appeared, and I could keep casting without fizzling. The sigil does seem to last far longer than the usual 3-8 hours, so this may just be a visual bug. In comparison, a blue 30 charge sigil got 3 pips after about 3 casts. And a gray 18 charge sigil got 1 pip after 2 casts. Any further casts seem to always sizzle, indicating that it's been extended to the limit.
Reproduction Steps:

1.  Make a yellow sigil with 30 charge.
2.  Try to extend it with a 18 charge gray crystal for 5-10 times at least.
3.  Behold no pips on the sigil.