AppleScript error: Can't make (handler) into type string

If you ever get the following error message in an AppleScript program (at least while editing the program in the ScriptEditor):

Can't make (handler) into type string

fear not, it seems to be pretty simple to cure. In my case I got this more-specific error message:

Can't make (handler getDayOfWeek) into type string

This error just means that I tried to call a function like this:

say "today is " & getDayOfWeek

when I should have called it like this, including the parentheses at the end of the function name:

say "today is " & getDayOfWeek()

That's it, forgetting those parentheses was the whole cause of the error message.