Using group functions in evaluations

[akvakh]

Hello.

I have a strange issue with grpsum function in Zabbix 2.4 and 3.0.
Following the manuals, I've created following expression:
grpsum["Database servers","db.sipphones",last,0]

But it constantly displays the following error:
Cannot evaluate expression: expected numeric token at "grpsum["Database servers","db.sipphones",last,0]".
What am I doing wrong?

BR,
Andrey.