This crash happens on more than one computer, all of which are running

First I properly create the tooltip object and send TTM_TRACKACTIVATE.
Then on every mouse move, a TTM_TRACKPOSITION is sent.
This works fine, except that sometimes, extremely rarely, when sending
the TRACKPOSITION message, my application crashes with this call stack:

0012f284 5d459305 comctl32!DoShowBubble+0xc2
0012f3a0 77d18734 comctl32!ToolTipsWndProc+0x969
0012f3cc 77d18816 user32!InternalCallWinProc+0x28
0012f434 77d1c63f user32!UserCallWinProcCheckWow+0x150
0012f464 77d1e905 user32!CallWindowProcAorW+0x98
0012f484 7c16fcdb user32!CallWindowProcA+0x1b
0012f4a4 7c16e0c7 mfc71!CWnd:efWindowProcA+0x42
0012f4c0 7c16e14f mfc71!CWnd::WindowProc+0x39
0012f520 7c16e1b8 mfc71!AfxCallWndProc+0x91
0012f540 7c16e1f6 mfc71!AfxWndProc+0x46
0012f56c 77d18734 mfc71!AfxWndProcBase+0x39
0012f598 77d18816 user32!InternalCallWinProc+0x28
0012f600 77d1b4c0 user32!UserCallWinProcCheckWow+0x150
0012f654 77d1b50c user32!DispatchClientMessage+0xa3
0012f67c 7c91eae3 user32!__fnDWORD+0x24
0012f6a0 77d194be ntdll!KiUserCallbackDispatcher+0x13
0012f6dc 77d2f3e3 user32!NtUserMessageCall+0xc
0012f6fc 0072c307 user32!SendMessageA+0x7f
0012f72c 00725a4f (My Code)

Anyone have a clue?
Thank you!