My Shiny app deploys to RStudio Connect but fails to run - Error in value[[3L]](cond) : argument 9 is empty

My deployment to RStudio Connect reports in RStudio as completed, i.e.,

----- Deployment log finished at 2023-01-04 16:45:13 -----
Deployment completed: https://connect.XXXXXX

, but fails to run with the following error msg from the application log:

...
2023/01/04 15:24:01.565772195
2023/01/04 15:24:01.617045754 INFO [2023-01-04 15:24:01] Initializing app...
2023/01/04 15:24:01.787167142 Error in value[3L] : argument 9 is empty
2023/01/04 15:24:01.787187283 Calls: local ... tryCatch -> tryCatchList -> tryCatchOne ->
2023/01/04 15:24:01.787226073 Shiny application exiting ...
2023/01/04 15:24:01.787242333 Execution halted

This turned out to be an R vs. Python issue. I had constructed a list in R that contained a comma after the last entry -- allowed in Python (my usual language), but apparently not in R.