diff --git a/include/ucore/slot_allocator.h b/include/ucore/slot_allocator.h
index b6d88da..55d4eb9 100644
--- a/include/ucore/slot_allocator.h
+++ b/include/ucore/slot_allocator.h
@@ -16,10 +16,11 @@ struct UCSlotArena {
     uint8_t *memory_start;
     size_t memory_len;
     size_t n_slots;
+    size_t used_slots;
     size_t slot_size;
 };
 
-/** 
+/**
 * Initialize a memory arena for dynamically allocating slots of memory.
 * Each allocation returns memory slots of the same size
 * Note: For max speed, align @memory and @slot_size to the size of a pointer
@@ -53,26 +54,44 @@ static inline void uc_slot_arena_init(UCSlotArena *arena, void *memory, size_t m
     arena->slot_size = slot_size;
 }
 
+/**
+ * Allocate a slot.
+ *
+ * @return pointer to a slot, NULL if all slots are already allocated
+ */
 static inline void *uc_slot_alloc(UCSlotArena *arena)
 {
     UCSlot *slot = arena->free_list;
 
     if (slot != NULL) {
         arena->free_list = slot->next;
+        arena->used_slots++;
     }
 
     return slot;
 }
-
-static inline void uc_slot_free(UCSlotArena *arena, void *mem)
+/**
+ * Free a slot.
+ *
+ * @param mem pointer to a slot that previously was allocated with uc_slot_alloc
+ */
+static inline void uc_slot_free(UCSlotArena *arena, void *memory)
 {
-    if (mem != NULL) {
-        assert((uint8_t *)mem >= arena->memory_start);
-        assert((uint8_t *)mem < arena->memory_start + arena->memory_len);
-
-        UCSlot *slot = (UCSlot *)mem;
+    if (memory != NULL) {
+#ifdef DEBUG
+        for (const UCSlot *slot = arena->free_list ; slot = slot->next) {
+            if (slot == mem) {
+                assert("Double free detected");
+            }
+        }
+#endif
+        assert((uint8_t *)memory >= arena->memory_start);
+        assert((uint8_t *)memory < arena->memory_start + arena->memory_len);
+        assert(arena->used_slots > 0);
+        UCSlot *slot = (UCSlot *)memory;
         slot->next = arena->free_list;
         arena->free_list = slot;
+        arena->used_slots--;
     }
 }