Create two calculated items. The first one will have expression like:

-1

and the second one will reference the first item like:

-last(host:key1)

Type of information: Numeric (float) for both of them.

The second item becomes unsupported with the following error:

Cannot evaluate expression: expected numeric token at "-1".

In the process of expression evaluation server simply substitutes trigger functions with their value without putting them in parentheses, which can sometimes produce invalid expressions (like "--1" in our example) for further evaluation stages.