Error with IsNull function

Hi Team,

We have a requirement to set Vesting Period based on Person's Probation Period. If a person doesn't have any Probation Period, then there should be no Vesting Period for that person. To achieve this, I have used below formula.

But while compiling the formula, I'm getting below error.

"line 20, either function IS_NULL doesn't exist or the combination of parameter types doesn't match the statement."

What is the mistake I'm doing here and how can I overcome this error?

Thanks in advance!

Code snippet:

Default for PER_ASG_PROBATION_PERIOD is 0

Default for PER_ASG_PROBATION_UNIT is 'A'