Alerting on derivative value - returning "null"

[michbsd]

Hi,

I am trying to create a threshold alert on a derivative value.

This is my alert:

When I test the rule, I get:

Why am I getting Null values?

Thanks,

[michbsd]

This is my query:

[michbsd]

I have tried playing around with the interval values (when I set the dashboard to show last 5 minutes, it works):

[michbsd]

Can anyone offer insights??

thanks