diff --git a/agent-shell-heartbeat.el b/agent-shell-heartbeat.el
index 0474dd41..1a572a4f 100644
--- a/agent-shell-heartbeat.el
+++ b/agent-shell-heartbeat.el
@@ -84,12 +84,12 @@ Returns the heartbeat alist with the timer started."
       (map-put! heartbeat :heartbeat-timer
                 (run-at-time 0 (/ 1.0 (map-elt heartbeat :beats-per-second))
                              (lambda ()
-                               (map-put! heartbeat :status 'busy)
-                               (map-put! heartbeat :value
-                                         (1+ (map-elt heartbeat :value)))
-                               (funcall (map-elt heartbeat :on-heartbeat)
-                                        (map-elt heartbeat :value)
-                                        (map-elt heartbeat :status))))))
+                               (when-let ((value (map-elt heartbeat :value)))
+                                 (map-put! heartbeat :status 'busy)
+                                 (map-put! heartbeat :value (1+ value))
+                                 (funcall (map-elt heartbeat :on-heartbeat)
+                                          (map-elt heartbeat :value)
+                                          (map-elt heartbeat :status)))))))
     heartbeat))
 
 (cl-defun agent-shell-heartbeat-stop (&key heartbeat)
diff --git a/agent-shell.el b/agent-shell.el
index 6f8bee80..d6d90a0a 100644
--- a/agent-shell.el
+++ b/agent-shell.el
@@ -5176,9 +5176,9 @@ See https://agentclientprotocol.com/protocol/session-modes for details."
                         ('dots-round '("⢎⡰" "⢎⡡" "⢎⡑" "⢎⠱" "⠎⡱" "⢊⡱" "⢌⡱" "⢆⡱"))
                         ('wide '("░   " "░░  " "░░░ " "░░░░" "░░░ " "░░  " "░   " "    "))
                         ((pred listp) agent-shell-busy-indicator-frames)
-                        (_ '("▁" "▂" "▃" "▄" "▅" "▆" "▇" "█" "▇" "▆" "▅" "▄" "▃" "▂")))))
-    (concat " " (seq-elt frames (mod (map-nested-elt (agent-shell--state) '(:heartbeat :value))
-                                     (length frames))))))
+                        (_ '("▁" "▂" "▃" "▄" "▅" "▆" "▇" "█" "▇" "▆" "▅" "▄" "▃" "▂"))))
+              (value (map-nested-elt (agent-shell--state) '(:heartbeat :value))))
+    (concat " " (seq-elt frames (mod value (length frames))))))
 
 (defun agent-shell--mode-line-format ()
   "Return `agent-shell''s mode-line format.