Error when using lubridate arithmetic in if_else loop

[chris-nunn]

Error in mutate():
i In argument: start = if_else(month == "dec", start - years(1), start).
Caused by error in vec_ptype_common():
! Column second (size 432) must match the data frame (size 1).
i In file slice.c at line 191.
i Install the winch package to get additional debugging info the next time you get this error.
i This is an internal error that was detected in the vctrs package.
Please report it at https://github.com/r-lib/vctrs/issues with a reprex and the full backtrace.

This is the backtrace:

x

1. +-... %>% ...
2. +-dplyr::mutate(...)
3. +-tidyr::pivot_longer(., future_suitability:historical)
4. +-dplyr::rename(., future_suitability = value)
5. +-dplyr::mutate(...)
6. +-dplyr:::mutate.data.frame(., start = if_else(month == "dec", start - years(1), start))
7. | -dplyr:::mutate_cols(.data, dplyr_quosures(...), by)
8. | +-base::withCallingHandlers(...)
9. | -dplyr:::mutate_col(dots[[i]], data, mask, new_columns)
10. | -mask$eval_all_mutate(quo)
11. | -dplyr (local) eval()
12. +-dplyr::if_else(month == "dec", start - years(1), start)
13. | -dplyr:::vec_case_when(...)
14. | -vctrs::vec_ptype_common(!!!everything, .ptype = ptype, .call = call)
15. -rlang:::stop_internal_c_lib(...)
16. -rlang::abort(message, call = call, .internal = TRUE, .frame = frame)`

[chris-nunn]

start, the variable being acted on, was not a date