Invalid access to memory - Vcl::Imaging::Pngimage::TPngImage::DrawPartialTrans

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WinSCP 6.3.6

Error message:
Invalid access to memory.

Stack trace:
(FFFFF001)WinSCP.exe
(000B03FF) ntdll.dll
(0007AB8A) ntdll.dll.KiUserExceptionDispatcher
(0000292D) GDI32.dll.DeleteObject
(0054731D) Vcl::Imaging::Pngimage::TPngImage::DrawPartialTrans

The problem happens rarely. I cannot reproduce it.