"if isnull" formula question

I have the following formula that I am using:

if isnull ({table.field}) then "" else "Quantity"

I am using the formula above and it is working exactly like I want it to for a majority of my fields but for a "number" type field the formula

must think that there is a quantity because the word "Quantity" keeps printing on my report even though no quantity actually exists for the

field itself?

Any thoughts?

Thank You.