diff --git a/include/jemalloc/internal/background_thread_inlines.h b/include/jemalloc/internal/background_thread_inlines.h
index ef50231e..f85e86fa 100644
--- a/include/jemalloc/internal/background_thread_inlines.h
+++ b/include/jemalloc/internal/background_thread_inlines.h
@@ -15,7 +15,12 @@ background_thread_enabled_set(tsdn_t *tsdn, bool state) {
 JEMALLOC_ALWAYS_INLINE background_thread_info_t *
 arena_background_thread_info_get(arena_t *arena) {
 	unsigned arena_ind = arena_ind_get(arena);
-	return &background_thread_info[arena_ind % ncpus];
+	return &background_thread_info[arena_ind % max_background_threads];
+}
+
+JEMALLOC_ALWAYS_INLINE background_thread_info_t *
+background_thread_info_get(size_t ind) {
+	return &background_thread_info[ind % max_background_threads];
 }
 
 JEMALLOC_ALWAYS_INLINE uint64_t
diff --git a/src/ctl.c b/src/ctl.c
index 1e713a3d..0eb8de13 100644
--- a/src/ctl.c
+++ b/src/ctl.c
@@ -2044,9 +2044,8 @@ arena_reset_prepare_background_thread(tsd_t *tsd, unsigned arena_ind) {
 	if (have_background_thread) {
 		malloc_mutex_lock(tsd_tsdn(tsd), &background_thread_lock);
 		if (background_thread_enabled()) {
-			unsigned ind = arena_ind % ncpus;
 			background_thread_info_t *info =
-			    &background_thread_info[ind];
+			    background_thread_info_get(arena_ind);
 			assert(info->state == background_thread_started);
 			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
 			info->state = background_thread_paused;
@@ -2059,9 +2058,8 @@ static void
 arena_reset_finish_background_thread(tsd_t *tsd, unsigned arena_ind) {
 	if (have_background_thread) {
 		if (background_thread_enabled()) {
-			unsigned ind = arena_ind % ncpus;
 			background_thread_info_t *info =
-			    &background_thread_info[ind];
+			    background_thread_info_get(arena_ind);
 			assert(info->state == background_thread_paused);
 			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
 			info->state = background_thread_started;
diff --git a/test/unit/arena_reset.c b/test/unit/arena_reset.c
index f5fb24d1..c1ccb097 100644
--- a/test/unit/arena_reset.c
+++ b/test/unit/arena_reset.c
@@ -142,7 +142,7 @@ do_arena_reset_post(void **ptrs, unsigned nptrs, unsigned arena_ind) {
 
 	if (have_background_thread) {
 		malloc_mutex_lock(tsdn,
-		    &background_thread_info[arena_ind % ncpus].mtx);
+		    &background_thread_info_get(arena_ind)->mtx);
 	}
 	/* Verify allocations no longer exist. */
 	for (i = 0; i < nptrs; i++) {
@@ -151,7 +151,7 @@ do_arena_reset_post(void **ptrs, unsigned nptrs, unsigned arena_ind) {
 	}
 	if (have_background_thread) {
 		malloc_mutex_unlock(tsdn,
-		    &background_thread_info[arena_ind % ncpus].mtx);
+		    &background_thread_info_get(arena_ind)->mtx);
 	}
 
 	free(ptrs);