Maybe I'm missing something here...

I've had this code for years now, and for some reason it's like as if it's got an extra return in it.

(DEFUN C:FF ()

(COMMAND "FILLET" "R" "0")

(PRINC))

Now I've never used it on 2016, maybe that's it???

i dont know 2016 but maybe try (setvar "filletrad" 0)?

gr. Rlx

Welcome to CADTutor.

Isn't the default setting ?

FWIW, you can apply a zero-radius fillet using shift-click.

It's acting like there is an exra return....

As suggested above, use:

(defun c:ff nil (setvar 'filletrad 0.0) (princ))

My $0.05

(defun C:r0 () (setvar "filletrad" 0.0)(command "fillet" )) ;sets fillet radius to zero
(defun c:r6 () (setvar "filletrad" 7)(command "fillet") )
(defun c:r7 () (setvar "filletrad" 7)(command "fillet") )
(defun c:r8 () (setvar "filletrad" (command "fillet") )
(defun c:r9 () (setvar "filletrad" 9)(command "fillet") )

This may also be useful: Custom Fillet Commands.