diff --git a/cutil.c b/cutil.c
index e9f1696..4030b98 100644
--- a/cutil.c
+++ b/cutil.c
@@ -20,7 +20,7 @@ void* _cu_realloc(void* mem, size_t size)
 
 void _cu_free(void* mem)
 {
-    return heap_free((mblock_t*)((u8*)mem - sizeof(mblock_t)));
+    return heap_free(&cu_heap, (mblock_t*)((u8*)mem - sizeof(mblock_t)));
 }
 
 void* (*cu_malloc)(size_t) = _cu_malloc;
diff --git a/heap.c b/heap.c
index 1663178..a415e05 100644
--- a/heap.c
+++ b/heap.c
@@ -55,14 +55,15 @@ mblock_t* heap_alloc(mheap_t* heap, uword _size)
     return block;
 }
 
-void heap_free(mblock_t* block)
+void heap_free(mheap_t* heap, mblock_t* block)
 {
     block->size &= ~MBLOCK_ALLOCATED;
     mblock_t* prev = block->prev;
     mblock_t* next = (mblock_t*)((u8*)block + block->size);
-    if (!(prev->size & MBLOCK_ALLOCATED))
+    mblock_t* end = (mblock_t*)((u8*)heap->start + heap->size);
+    if (prev && !(prev->size & MBLOCK_ALLOCATED))
         heap_join(block, prev);
-    if (!(next->size & MBLOCK_ALLOCATED))
+    if (next < end && !(next->size & MBLOCK_ALLOCATED))
         heap_join(block, next);
 }
 
@@ -81,7 +82,7 @@ mblock_t* heap_realloc(mheap_t* heap, mblock_t* block, uword _size)
     }
     else
     {
-        heap_free(block);
+        heap_free(heap, block);
         new = heap_alloc(heap, size);
         if (new)
         {
diff --git a/heap.h b/heap.h
index 0bd6300..6cd3a01 100644
--- a/heap.h
+++ b/heap.h
@@ -26,7 +26,7 @@ void heap_init(mheap_t* heap, void* start, uword size);
 void heap_split(mblock_t* block, uword size);
 void heap_join(mblock_t* block, mblock_t* other);
 mblock_t* heap_alloc(mheap_t* heap, uword _size);
-void heap_free(mblock_t* block);
+void heap_free(mheap_t* heap, mblock_t* block);
 mblock_t* heap_realloc(mheap_t* heap, mblock_t* block, uword _size);
 
 #endif
\ No newline at end of file
diff --git a/test.c b/test.c
index 68a1adf..763d490 100644
--- a/test.c
+++ b/test.c
@@ -207,7 +207,7 @@ int main()
     void* m1 = heap_alloc(&heap, 8);
     void* m2 = heap_alloc(&heap, 32);
     void* m3 = heap_alloc(&heap, 9);
-    heap_free(m2);
+    heap_free(&heap, m2);
     void* m4 = heap_alloc(&heap, 8);
     printf("heap_alloc\t%p\n", m1);
     printf("heap_alloc\t%p\n", m2);