Pooled incident rate ratio (IRR) not estimated while using STIR command

[Sakshi Rajatbhai Tewari]

Hello, i was using stir

stir gender2, strata(strain_type) pool

and noticed that while the Crude and M-H comined IRRs were estimated, the "Pooled (direct)" IRR was ".". Am i doing something wrong for stata to not estimate this IRR?