Pet Mixer from item mall

i buy a pet mix from item mall, but it cant use for pink bird egg 5th pet, and in the quick info from item mall it say:
"Pet Mixer
Do you have a pet that not right for you?
Then, choose the correct pet!
You can mix two 5th stage pets to
have a new 4th stage pet of the same type.

Combine two 5th stage pets into one 4th stage pet

"?Caution
You will receive the same type of pet within the category.
Cash pets such as Lobster and General Crab cannot be used this item"
You can select only one 4th stage pet, So, please choose carefully.
Once used, item will start to expire regardless of your login time."

so didn't mention that cant use for pink bird egg, i try to put my both yellow sweet love-birds on the pet mixer...the system say pet not available. Please fix it...the pet mixer cost me 2000RPs sir.