Game crashing when launching in VR mode

Whenever I try launching my game in VR mode it crashes and gives me this error;

Assertion failed: IsValid() [File:D:\Build++UE4\Sync\Engine\Source\Runtime\Core\Public\Templates/SharedPointer.h] [Line: 890]

UE4Editor_Core
UE4Editor_Core
UE4Editor_SteamVR
UE4Editor_D3D11RHI
UE4Editor_D3D11RHI
UE4Editor_D3D11RHI
UE4Editor_RHI
UE4Editor_SlateRHIRenderer
UE4Editor_SlateRHIRenderer
UE4Editor_SlateRHIRenderer
UE4Editor_Core
UE4Editor_Core
UE4Editor_RenderCore
UE4Editor_RenderCore
UE4Editor_Core
UE4Editor_Core
kernel32
ntdll