#VALUE error

Greetings,

I have the following function which seems to run fine when I enter the calculated cell that calls the function and hit enter, however, if I do not enter the cell, I get the notorius #VALUE error, a value used in the formula is of the wrong data type. I basically have a table with 3 colums, Column A is the current date, Column B is a tracking number which consists of eight numerals and one alpha character, and column C is just a true or false. from a cell in column D i have entered =CountUniqueValues(a2:a501), which should give me the total number of unique dates, then in another cell in column D I have =CountUniqueValues(b2:b501), which should return the total number of unique tracking numbers entered. The calculation on column A works fine, but it gives me the #VALUE for Column B. Column A is formatted as DATE and Column B is formatted as TEXT.

Below is my function.

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Any help would be greatly appreciated.