`rlang` defines (and `purrr` reexports) many `is_*` functions, such as `is_list`, that produce weird interactions with `simmer`. Example (reported [here](https://groups.google.com/d/msg/simmer-devel/cgV3aLEyR5k/_FF7pGnLBQAJ)): ```r library(purrr) library(simmer) monitor_csv() #> Error: monitor_delim: 'args' is not a valid list ```