Proc phreg

[LEESK]

I am using phreg to analyze the my data.
However, the control frequency of the last group among the four groups is '0'.
However, the hazard ratio is calculated in the phreg
I do not know why the hazard ratio is calculated for any reason.

I try to write a paper using this result.
I do not know what to do.

[WillZh]

Your last group still has 4 cases... Its frequency is not zero but  rounded to 0 by excel