Meaning of negative value of MessageBoxA hWnd argument

I'm analyzing a malware, where I observed the malware launching a MessageBox with negative value of hWnd (FFFFFFF7h). Message box arguments: (Source:MSDN) int WINAPI MessageBox( _In_opt_ HWND hWnd, _In_opt_ LPCTSTR lpText, _In_opt_ LPCTSTR lpCaption, _In_ UINT uType ); hWnd stands for owner window, I wasn't able to find what it means when hWnd argument is negative. Any help is very much appreciated.