time constraint 'T' for custom infotype

HI experts,

Im creating a custom infotype and i want to assign time constraint 'T' for that infotype. But while assigning the time constraint in Infotype characteristcs in PM01, it throws the below error message.

"TIme constraint T is not allowed for 9021 infotype".

Pls tell me what is the pbm here and how to solve the problem?