diff --git a/debug_counter.c b/debug_counter.c
index b9c32c1d15..f17fb1c488 100644
--- a/debug_counter.c
+++ b/debug_counter.c
@@ -51,6 +51,7 @@ rb_debug_counter_reset(void)
           case RB_DEBUG_COUNTER_mjit_length_unit_queue:
           case RB_DEBUG_COUNTER_mjit_length_active_units:
           case RB_DEBUG_COUNTER_mjit_length_compact_units:
+          case RB_DEBUG_COUNTER_mjit_length_stale_units:
             // These counters may be decreased and should not be reset.
             break;
           default:
diff --git a/debug_counter.h b/debug_counter.h
index 8510bb90f8..d15330f559 100644
--- a/debug_counter.h
+++ b/debug_counter.h
@@ -280,6 +280,7 @@ RB_DEBUG_COUNTER(mjit_cancel_trace)
 RB_DEBUG_COUNTER(mjit_length_unit_queue)
 RB_DEBUG_COUNTER(mjit_length_active_units)
 RB_DEBUG_COUNTER(mjit_length_compact_units)
+RB_DEBUG_COUNTER(mjit_length_stale_units)
 
 /* Other MJIT counters */
 RB_DEBUG_COUNTER(mjit_compile_failures)
diff --git a/mjit_worker.c b/mjit_worker.c
index 06d259bc06..be0910b611 100644
--- a/mjit_worker.c
+++ b/mjit_worker.c
@@ -334,6 +334,7 @@ add_to_list(struct rb_mjit_unit *unit, struct rb_mjit_unit_list *list)
     (void)RB_DEBUG_COUNTER_INC_IF(mjit_length_unit_queue, list == &unit_queue);
     (void)RB_DEBUG_COUNTER_INC_IF(mjit_length_active_units, list == &active_units);
     (void)RB_DEBUG_COUNTER_INC_IF(mjit_length_compact_units, list == &compact_units);
+    (void)RB_DEBUG_COUNTER_INC_IF(mjit_length_stale_units, list == &stale_units);
 
     list_add_tail(&list->head, &unit->unode);
     list->length++;
@@ -346,6 +347,7 @@ remove_from_list(struct rb_mjit_unit *unit, struct rb_mjit_unit_list *list)
     rb_debug_counter_add(RB_DEBUG_COUNTER_mjit_length_unit_queue, -1, list == &unit_queue);
     rb_debug_counter_add(RB_DEBUG_COUNTER_mjit_length_active_units, -1, list == &active_units);
     rb_debug_counter_add(RB_DEBUG_COUNTER_mjit_length_compact_units, -1, list == &compact_units);
+    rb_debug_counter_add(RB_DEBUG_COUNTER_mjit_length_stale_units, -1, list == &stale_units);
 #endif
 
     list_del(&unit->unode);