Currently, the expression parser returns the position at which the expression contains an unexpected or invalid token.

For example, when try to add this expression:

25 + sum(last(/host/key1, #1), last(/host/key1, #2), 1a)

we will get this error:

Invalid parameter "/1/expression": incorrect expression starting from "sum(last(/host/key1, #1), last(/host/key1, #2), 1a)". 

The parser points to this token because it contains an invalid expression in the third parameter. It would be more correct to point to the parameter that contains the error with friendly message like "incorrect numeric value starting from "1a)""