Custom node with the following code View.ExposureScale Error

I want normalize value then lerp opacity so that in darker areas, the opacity! decreases.

[SM5] /Engine/Generated/Material.ush(1414,8-25): error X3004: undeclared identifier ‘View_ExposureScale’ View.ExposureScale Error
[SM5] /Engine/Generated/Material.ush(1414,1-26): error X3080: ‘CustomExpression0’: function must return a value
Material Expression Error in 4.19.3 on compiling shaders

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Thanks

Also have this problem, I guess the param has been moved (again).

Would love to know where these are kept so we can easily find changes!