isnull() function returns false when input is 0

Hi everyone,

I was doing Lesson 5 Challenge in the Developer course that has an if() function and the condition to be tested is isnull(ri!test). So my expression looks like this:

if(
isnull(ri!test), "null", ri!test
)

Now my issue is, whether the input I put in is 0 or not, it always returns the ri!test.

I tested the isnull() separately, and it doesn't return True if the value i pass is 0. Can someone help if I'm doing something wrong here?

All I can think of is that 0 is not being read as a null input. 

when I specifically type in isnull(null) , it returns True. But when I type in isnull(0) and test, it gives a False.

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