diff --git a/headers/os/support/BlockCache.h b/headers/os/support/BlockCache.h
index 7f8c5c331a..958951376b 100644
--- a/headers/os/support/BlockCache.h
+++ b/headers/os/support/BlockCache.h
@@ -58,12 +58,7 @@ private:
 				BBlockCache(const BBlockCache &);
 	BBlockCache	&operator=(const BBlockCache &);
 
-	struct _FreeBlock
-	{
-		uint32		magic1;
-		_FreeBlock *next;
-		uint32		magic2;
-	};
+	struct _FreeBlock;
 
 	_FreeBlock *fFreeList;
 	size_t		fBlockSize;
diff --git a/src/kits/support/BlockCache.cpp b/src/kits/support/BlockCache.cpp
index d369b715aa..d905ca85cc 100644
--- a/src/kits/support/BlockCache.cpp
+++ b/src/kits/support/BlockCache.cpp
@@ -29,6 +29,13 @@
 #define MAGIC1		0x9183f4d9
 #define MAGIC2		0xa6b3c87d
 
+struct BBlockCache::_FreeBlock {
+	DEBUG_ONLY(	uint32		magic1;	)
+				_FreeBlock *next;
+	DEBUG_ONLY(	uint32		magic2;	)
+};
+
+
 // The requirements set by the BeBook's description of the destructor,
 // as well as Get() function, allowing the caller to dispose of the
 // memory, do not allow to allocate one large block to be used as pool.