I’m trying to set up data validation on a cell in my Google Sheets document to make sure it’s not left empty. I created a custom validation rule with the formula =ISBLANK(C4) where C4 is the target cell.
The problem is that the validation works backwards. When the cell is empty, no validation error shows up. But when I enter data into the cell, it triggers a validation error. This is completely opposite to what I expected.
I tested the same ISBLANK(C4) formula in another cell and it works perfectly. It returns TRUE for empty cells and FALSE when there’s content. I also tried using LEN(C4) which gives 0 for empty cells and shows the right character count otherwise.
The cell doesn’t have any formulas in it, but it’s referenced in a SUM function elsewhere in my sheet. The cell is set to currency format. Any ideas what might be causing this weird behavior?
ah yeah this caught me off guard too when i first started using sheets validation. basically google sheets validation logic is backwards from what feels intuitive - your formula needs to return true for VALID data, not invalid data. so when you use ISBLANK(C4), it returns true for empty cells which sheets interprets as “this is valid” instead of “this should trigger an error”. just wrap it with NOT() function and you’ll be good to go.
This is a common confusion with Google Sheets data validation logic. I’ve seen this trip up many users because the validation formula works differently than you’d expect. The key thing to understand is that your validation formula needs to return TRUE when the data is valid and FALSE when it should trigger an error. Right now your ISBLANK(C4) formula returns TRUE for empty cells, which tells Sheets that empty cells are valid - hence no error appears. When you enter data, ISBLANK returns FALSE, making Sheets think the data is invalid and triggering the error. You have two options to fix this: either use NOT(ISBLANK(C4)) as your validation formula, or switch to a different validation type like ‘Custom formula is’ with a formula that returns TRUE for valid entries. I usually prefer the NOT approach since it’s more straightforward.
I ran into this exact same issue a few months back and it drove me crazy until I figured out what was happening. The problem is with how Google Sheets interprets the validation logic. When you use ISBLANK(C4) as your validation formula, Sheets considers the validation to PASS when the formula returns TRUE and FAIL when it returns FALSE. Since ISBLANK returns TRUE for empty cells, your validation is essentially saying “this cell is valid when it’s blank” which is the opposite of what you want. To fix this, you need to flip the logic by using NOT(ISBLANK(C4)) instead. This way the validation will return TRUE when the cell has content and FALSE when it’s empty, triggering the error message only when the cell is actually blank. The currency formatting and SUM reference shouldn’t affect this behavior at all.