Invalid access to memory - _pq_base_concatenate

I just typed in the hostname, hit Enter, typed in the username, hit Enter, pasted in the password, and hit Enter again. I closed the tab to end the session and started it over and it worked fine.

WinSCP 6.3.6

Error message:
Invalid access to memory.

Stack trace:
(00AB36CB) _pq_base_concatenate
(00E24D56) C6612_3
(00E26DBE) ____ExceptionHandler
(00E23B78) __ExceptionHandler
(00087F7F) ntdll.dll
(00074501) ntdll.dll.KiUserExceptionDispatcher
(00AC5CE3) _ssh2transport_finalise_exhash
(00AC5F05) _ssh2transport_finalise_exhash
(00A6DF2A) C3743_0
(00A6E01D) _run_toplevel_callbacks
(00BCBE09) TSecureShell::EventSelectLoop
(00BCC445) TSecureShell::Idle
(00C5D8BB) TTerminal::Idle
(000C5F69) TTerminalManager::Idle
(0001C62A) TCustomScpExplorerForm::Idle
(00055027) TNonVisualDataModule::DoIdle
(00054FE4) TNonVisualDataModule::SessionIdleTimerTimer
(00226F74) System::Classes::_18201
(000406A9) USER32.dll
(00036E15) USER32.dll
(0003574C) USER32.dll
(0003526B) USER32.dll.DispatchMessageW

(00E253C1) C6612_3
(00E2699B) _ReThrowException
(000C5FB2) TTerminalManager::Idle
(0001C62A) TCustomScpExplorerForm::Idle
(00055027) TNonVisualDataModule::DoIdle
(00054FE4) TNonVisualDataModule::SessionIdleTimerTimer
(00226F74) System::Classes::_18201
(000406A9) USER32.dll
(00036E15) USER32.dll
(0003574C) USER32.dll
(0003526B) USER32.dll.DispatchMessageW

The problem happens rarely. I cannot reproduce it.