applyDevelopSettings fails: ?:0: attempt to call up value '?' (a nil value)

I am trying to write a first Lightroom plugin to automatically crop and rotate images.  I should be able to work out the crop settings (assuming I can translate to LR values later, and avoid limitations on setting crop values from the SDK), but I'm getting stuck at a more basic level applying any develop adjustment at all to any image.  Being new to LUA does not help

Whether I use something like photo:applyDevelopSettings({Exposure = 0.1}) or LrDevelopController.setValue("CropAngle", "0.1"), in both cases this fails with:

         ?:0: attempt to call up value '?' (a nil value)

Am I missing something in the way this call is formatted?  Or do I need a full set of settings from getDevelopSettings(), or a preset or something else?  In case it makes a difference, the target photos do not have any existing develop settings beyond import.

My full plugin that fails:

-- Access the Lightroom SDK namespaces.

local LrApplication       = import 'LrApplication'

local LrDevelopController = import 'LrDevelopController'

local LrTasks             = import 'LrTasks'

local LrDialogs           = import 'LrDialogs'

function MyPluginTopLevel()

  -- Main top level callback

  LrTasks.startAsyncTask(MyPluginTask, "MyPlugin main task")

end

function MyPluginTask()

  local catalog = LrApplication.activeCatalog()

  catalog:withWriteAccessDo("MyPlugin test", MyPluginApplyParams, {timeout = 10})

end

local function MyPluginApplyParams(context)

  local catalog        = LrApplication.activeCatalog()

  local selectedPhoto  = catalog:getTargetPhoto()

  if (selectedPhoto == nil) then

      LrDialogs.message( "Error: cannot get selected photo", nil, "info" );

      return nil

  else

     -- EITHER OF THESE TWO NEXT LINES FAILS:

     -- LrDevelopController.setValue("CropAngle", "0.1")

     selectedPhoto:applyDevelopSettings({CropAngle = "0.1"})

  end

end

-- Call top level:

MyPluginTopLevel()