Function scalar_rejection_reason 

#[doc(hidden)]
pub fn scalar_rejection_reason(value: SEXP) -> String

Why a value that Rf_asInteger / Rf_asReal / Rf_asLogical (or a raw coercion) could not reduce to one non-NA scalar was refused, in R terms: got NULL, got length 0, got "abc" (a string that does not parse), NA is not allowed, or got <type> for a value of another kind.

The reason of a sidecar scalar setter’s argument error ('value' must be a number: got "abc", #1594). Those setters read the value with the Rf_as* coercions, which report failure as NA with no error value, so the reason is worded from the rejected value itself.