I have managed to track down the issue to these changes betwen two versions of swank-allegro.lisp (defmethod debugger-info-for-emacs (start end) - (list (debugger-condition-for-emacs) + (list (format-condition-for-emacs) (format-restarts-for-emacs) (backtrace start end))) (defmethod frame-locals (index) (let ((frame (nth-frame index))) (loop for i from 0 below (debugger:frame-number-vars frame) - collect (list :name (to-string (debugger:frame-var-name frame i)) + collect (list :symbol (debugger:frame-var-name frame i) :id 0 :value-string (to-string (debugger:frame-var-value frame i)))))) If i am not changing the file everything works perfectly yet after applying changes to the function frame-locals slime puts nil's instead of variable names. And modifying debugger-info-for-emacs effectively disables the debuger ... Ignas Mikalajunas