diff --git a/m-bptree.h b/m-bptree.h
index 79fc631b..42914a32 100644
--- a/m-bptree.h
+++ b/m-bptree.h
@@ -1016,6 +1016,7 @@
   M_C(name, _next)(it_t it)                                                   \
   {                                                                           \
     M_ASSERT (it != NULL && it->node != NULL);                                \
+    M_ASSERT (M_C(name, _is_leaf)(it->node));                                 \
     it->idx ++;                                                               \
     if (it->idx >= -it->node->num && it->node->next != NULL) {                \
       it->node = it->node->next;                                              \
@@ -1027,6 +1028,7 @@
   M_C(name, _ref)(it_t it)                                                    \
   {                                                                           \
     M_ASSERT (it != NULL && it->node != NULL);                                \
+    M_ASSERT (M_C(name, _is_leaf)(it->node));                                 \
     M_ASSERT (it->idx <= -it->node->num);                                     \
     M_IF(isMap)(                                                              \
                 it->pair.key_ptr = &it->node->key[it->idx];                   \
@@ -1089,7 +1091,7 @@
   M_C(name, _min)(const tree_t b)                                             \
   {                                                                           \
     M_BPTR33_CONTRACT(N, isMulti, key_oplist, b);                             \
-    M_ASSERT (b->size > 0);                                                   \
+    if (M_UNLIKELY (b->size == 0)) return NULL;                               \
     node_t n = b->root;                                                       \
     /* Scan down the nodes */                                                 \
     while (!M_C(name, _is_leaf)(n)) {                                         \
@@ -1102,6 +1104,7 @@
   M_C(name, _max)(const tree_t b)                                             \
   {                                                                           \
     M_BPTR33_CONTRACT(N, isMulti, key_oplist, b);                             \
+    if (M_UNLIKELY (b->size == 0)) return NULL;                               \
     node_t n = b->root;                                                       \
     /* Scan down the nodes */                                                 \
     while (!M_C(name, _is_leaf)(n)) {                                         \
diff --git a/tests/test-mbptree.c b/tests/test-mbptree.c
index 7fbdbde7..8f906647 100644
--- a/tests/test-mbptree.c
+++ b/tests/test-mbptree.c
@@ -350,6 +350,9 @@ static void test5(void)
   btree_t b;
   btree_init(b);
 
+  assert (btree_cmin(b) == NULL);
+  assert (btree_cmax(b) == NULL);
+
   for(int i = 0; i < 1000; i+=3)
     btree_set_at(b, i, 1000*i);
   for(int i = 1; i < 1000; i+=3)