the SETF needs to be wrapped, too.

src/body.lisp

@@ -89,8 +89,8 @@
                      (defun (setf ,external) (new-value body)
                        (setf (,internal body) new-value
                              (,cached body) (,wrapper new-value))))))
-  (wrap body-mass body-%mass body-inverse-mass /)
-  (wrap body-inertia body-%inertia body-inverse-inertia /)
+  (wrap body-mass body-%mass body-inverse-mass maybe-inverse)
+  (wrap body-inertia body-%inertia body-inverse-inertia maybe-inverse)
   (wrap body-angle body-%angle body-rotation angle->vec))
 
 (defgeneric body-update-velocity (body gravity damping dt)

src/package.lisp

@@ -8,6 +8,7 @@
            :ensure-list
            :clamp
            :maybe/
+           :maybe-inverse
            :fun :_
            :deletef
            :delete-iff

src/utils.lisp

@@ -21,10 +21,13 @@
 
 (declaim (inline maybe/)
          (ftype (function (double-float double-float) double-float) maybe/))
-(defun maybe/ (a b)
+(defun maybe/ (a &optional b)
   ;; Don't declare me (optimize speed), because that chokes SBCL
   (if (zerop b) 0d0 (/ a b)))
 
+(defun maybe-inverse (x)
+  (if (zerop x) 0d0 (/ x)))
+
 (defmacro fun (&body body)
   `(lambda (&optional _) (declare (ignorable _)) ,@body))