diff --git a/src/mono/mono/mini/interp/transform.c b/src/mono/mono/mini/interp/transform.c
index 814d5af9a8c9af..1065becef752c6 100644
--- a/src/mono/mono/mini/interp/transform.c
+++ b/src/mono/mono/mini/interp/transform.c
@@ -8553,7 +8553,7 @@ interp_mark_ref_slots_for_vt (TransformData *td, int base_offset, MonoClass *kla
 retry:
 		if (mini_type_is_reference (ftype) || ftype->type == MONO_TYPE_I || ftype->type == MONO_TYPE_U || m_type_is_byref (ftype)) {
 			int index = offset / sizeof (gpointer);
-			mono_bitset_set_fast (td->ref_slots, index);
+			mono_bitset_set (td->ref_slots, index);
 			if (td->verbose_level)
 				g_print ("Stack ref slot vt field at off %d\n", offset);
 		} else if (ftype->type == MONO_TYPE_VALUETYPE || ftype->type == MONO_TYPE_GENERICINST) {
@@ -8585,6 +8585,8 @@ interp_mark_ref_slots_for_var (TransformData *td, int var)
 	if (!td->ref_slots || max_index >= td->ref_slots->size) {
 		guint32 old_size = td->ref_slots ? (guint32)td->ref_slots->size : 0;
 		guint32 new_size = old_size ? old_size * 2 : 32;
+		while (new_size <= max_index)
+			new_size *= 2;
 
 		gpointer mem = mono_mempool_alloc0 (td->mempool, mono_bitset_alloc_size (new_size, 0));
 		MonoBitSet *new_ref_slots = mono_bitset_mem_new (mem, new_size, 0);
@@ -8602,7 +8604,7 @@ interp_mark_ref_slots_for_var (TransformData *td, int var)
 		// Managed pointers in interp are normally MONO_TYPE_I
 		if (mini_type_is_reference (type) || type->type == MONO_TYPE_I || type->type == MONO_TYPE_U || m_type_is_byref (type)) {
 			int index = td->vars [var].offset / sizeof (gpointer);
-			mono_bitset_set_fast (td->ref_slots, index);
+			mono_bitset_set (td->ref_slots, index);
 			if (td->verbose_level)
 				g_print ("Stack ref slot at off %d for var %d\n", index * sizeof (gpointer), var);
 		}