diff --git a/winum.el b/winum.el
index b5b98b7..f3fc5f7 100644
--- a/winum.el
+++ b/winum.el
@@ -39,8 +39,6 @@
 ;;
 ;;; Code:
 ;;
-;; FIXME: when `winum-scope' is changed from frame-local to non-local in
-;;        customize, the mode-line is messed up until next `winum-update'.
 ;; FIXME: The mode-line's window number is not always up to date in all frames.
 ;;
 
@@ -171,6 +169,10 @@ To get a number given a window, use the `cdr' of a value.
 
 Such a structure allows for per-frame bidirectional fast access.")
 
+(defvar winum--last-used-scope winum-scope
+  "Tracks the last used `winum-scope'.
+Needed to detect scope changes at runtime.")
+
 ;; Interactive functions -------------------------------------------------------
 
 ;;;###autoload
@@ -479,6 +481,7 @@ windows, however a higher number can be reserved by the user-defined
 
 (defun winum--set-window-vector (window-vector)
   "Set WINDOW-VECTOR according to the current `winum-scope'."
+  (winum--check-for-scope-change)
   (if (eq winum-scope 'frame-local)
       (puthash (selected-frame)
                (cons window-vector
@@ -489,6 +492,7 @@ windows, however a higher number can be reserved by the user-defined
 (defun winum--get-window-vector ()
   "Return the window vector used to get a window given a number.
 This vector is not stored the same way depending on the value of `winum-scope'."
+  (winum--check-for-scope-change)
   (if (eq winum-scope 'frame-local)
       (car (gethash (selected-frame) winum--frames-table))
     winum--window-vector))
@@ -497,6 +501,7 @@ This vector is not stored the same way depending on the value of `winum-scope'."
   "Return the numbers hashtable used to get a number given a window.
 This hashtable is not stored the same way depending on the value of
 `winum-scope'"
+  (winum--check-for-scope-change)
   (if (eq winum-scope 'frame-local)
       (cdr (gethash (selected-frame) winum--frames-table))
     winum--numbers-table))
@@ -520,6 +525,14 @@ using the `winum-assign-func', or using `winum-auto-assign-0-to-minibuffer'."
         (select-window window)
       (error "Got a dead window %S" window))))
 
+(defun winum--check-for-scope-change ()
+  "Check whether the `winum-scope' has been changed.
+If a change is detected run `winum--init' to reinitialize all
+internal data structures according to the new scope."
+  (unless (eq winum-scope winum--last-used-scope)
+    (setq winum--last-used-scope winum-scope)
+    (winum--init)))
+
 (push "^No window numbered .$"     debug-ignored-errors)
 (push "^Got a dead window .$"      debug-ignored-errors)
 (push "^Invalid `winum-scope': .$" debug-ignored-errors)