Help needed (Invalid hook handle)

[Abin]

I was writing a keyboard hooking dll, used ::SetWindowsHookEx to obtain a HHOOK, which was 0x0233078B in my case, but when I was done with the hook, for some odd reason ::UnhookWindowsHookEx returned FALSE, and ::GetLastError returned error code ERROR_INVALID_HOOK_HANDLE. I debugged the code and value of my HHOOK never changed, so how could the handle, 0x0233078B, obtained from a successful ::SetWindowsHookEx call, suddenly become an invalid handle? Any helps are appreciated.