Calculated column to show days open

Hi I have created a calculated column to display the number of days open for an issue, the only thing is it seems to be doing a calculation for a blank row and will return a value like 45702 for it and I need help trying to remove this. Here is the calculation I have in my calculated column which returns the correct value for the open and closed items -

Days Open = IF (
ISBLANK ( 'Issues Raised'[Date Closed] ),
TODAY () - 'Issues Raised'[Date],
'Issues Raised'[Date Closed] - 'Issues Raised'[Date]
)

How do I stop it calculating the blank line?