fwrite: Improve validation of the na argument (#7867)

* fwrite: Improve validation of the na argument.

* Simplify conversion to character.

* Add NEWS.

* preferred min-diff fix

* NEWS entries are 'Notes', not bug fixes

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

Author: mcol

NEWS.md

@@ -80,8 +80,6 @@
 
 18. `example(local=TRUE)` where the example uses `[.data.table` works again (e.g. `example(':=', package='data.table', local=TRUE, echo=FALSE)`), [#7855](https://github.com/Rdatatable/data.table/issues/7855) re-fixing [#2972](https://github.com/Rdatatable/data.table/issues/2972). Thanks @michaelChirico for the fix.
 
-19. `fread()` returns a clearer error message when `dec = NA` is used, [#7737](https://github.com/Rdatatable/data.table/issues/7737). Thanks @mcol for the report and the fix.
-
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).
@@ -100,6 +98,10 @@
 
 8. Clarified `fread()` documentation and vignette regarding the interaction between `keepLeadingZeros = TRUE` and automatic header detection, [#5405](https://github.com/Rdatatable/data.table/issues/5405). Thanks @clemenskuehn for the report and @venom1204 for updating the documentation.
 
+9. `fread()` returns a clearer error message when `dec = NA` is used, [#7737](https://github.com/Rdatatable/data.table/issues/7737). Thanks @mcol for the report and the fix.
+
+10. `fwrite()` returns a clearer error message when `na = data.frame()` is used, [#7866](https://github.com/Rdatatable/data.table/issues/7866). Thanks @mcol for the report and the fix.
+
 ## data.table [v1.18.4](https://github.com/Rdatatable/data.table/milestone/45) (6 May 2026)
 
 ### BUG FIXES

R/fwrite.R

@@ -15,7 +15,7 @@ fwrite = function(x, file="", append=FALSE, quote="auto",
            verbose=getOption("datatable.verbose", FALSE),
            encoding = "",
            forceDecimal = FALSE) {
-  na = as.character(na[1L]) # fix for #1725
+  na = as.character(na) # fix for #1725
   if (length(encoding) != 1L || !encoding %chin% c("", "UTF-8", "native")) {
     stopf("Argument 'encoding' must be '', 'UTF-8' or 'native'.")
   }

inst/tests/tests.Rraw

@@ -10018,6 +10018,7 @@ test(1676.1, fwrite(dt, f, na=NULL), error=base_messages$stopifnot("length(na) =
 fwrite(dt, f, na=NA)
 test(1676.2, fread(f), data.table(x=1:2, y=c(NA, "a")))
 unlink(f)
+test(1676.3, fwrite(dt, na=data.frame()), error=base_messages$stopifnot("length(na) == 1L"))
 
 # duplicate names in foverlaps #1730
 a = data.table(start = 1:5, end = 2:6, c2 = rnorm(10), c2 = rnorm(10), key=c("start","end"))