ymax doesn't apply to AC waveforms?

I am using 6.1.8-64b.500.9.

I ran a AC and transient and when I applied ymax(IT("/ITX_CCII_buf/ccii_inp")), I get a result. but when I applied ymax(IF("/ITX_CCII_buf/ccii_inp")),

ERROR (VIVA-3002):expression evaluation failed: Expression evaluates to nil
ERROR (VIVA-3002):expression evaluation failed: ymax(IF("/ITX_CCII_buf/ccii_inp"))

Why ymax() cannot work on IF or VF results, if I want to find the AC response peak, what function should I use?

Regards
monglebest