diff --git a/src/heap.cpp b/src/heap.cpp
index d0dd5e8d45..5ec5ae724d 100644
--- a/src/heap.cpp
+++ b/src/heap.cpp
@@ -33,7 +33,12 @@ const bool Verbose = false;
 const bool Verbose2 = false;
 const bool Debug = false;
 const bool DebugFixies = false;
+
+#ifdef NDEBUG
 const bool DebugAllocation = false;
+#else
+const bool DebugAllocation = true;
+#endif
 
 #define ACQUIRE(x) MutexLock MAKE_NAME(monitorLock_) (x)
 
@@ -423,7 +428,9 @@ class Segment {
   }
 
   void dispose() {
-    free(context, data, (footprint(capacity())) * BytesPerWord);
+    if (data) {
+      free(context, data, (footprint(capacity())) * BytesPerWord);
+    }
     data = 0;
     map = 0;
   }
@@ -1675,8 +1682,7 @@ void* tryAllocate(Context* c, unsigned size)
   ACQUIRE(c->lock);
 
   if (DebugAllocation) {
-    size = pad(size);
-    size += 2 * BytesPerWord;
+    size = pad(size) + 2 * BytesPerWord;
   }
 
   if (size + c->count < c->limit) {
@@ -1699,21 +1705,21 @@ void* tryAllocate(Context* c, unsigned size)
 void free(Context* c, const void* p, unsigned size) {
   ACQUIRE(c->lock);
 
-  expect(c->system, c->count >= size);
-
   if (DebugAllocation) {
-    memset(const_cast<void*>(p), 0xFE, size);
+    size = pad(size) + 2 * BytesPerWord;
 
-    size = pad(size);
+    memset(const_cast<void*>(p), 0xFE, size - (2 * BytesPerWord));
 
     p = static_cast<const uintptr_t*>(p) - 1;
 
     expect(c->system, static_cast<const uintptr_t*>(p)[0] == 0x22377322);
 
     expect(c->system, static_cast<const uintptr_t*>(p)
-           [1 + (size / BytesPerWord)] == 0x22377322);
+           [(size / BytesPerWord) - 1] == 0x22377322);
   }
 
+  expect(c->system, c->count >= size);
+
   c->system->free(p);
   c->count -= size;
 }