Interface can be treated as a new data type. I received the following response to my case about the inconsistent (wrong, IMHO) behavior of ISBLANK: Unexpected behavior with ISBLANK and CONTAINS functions in Visualforce Page, Why are video calls so tiring? Since using the TRIM() function inside of the ISBLANK() function returned the correct result, perhaps it's the use of variables representing zero-length strings that is broken, not the ISBLANK() function. */ The "" (empty string) is a string text value, but there is no character within it. What does multiple key combinations over a paragraph in the manual mean? Compare the text field to an empty string, e.g, Account.Name == ”. Sample Package.xml: