diff --git a/NEWS.md b/NEWS.md
index 7f8323d506..fa2ca3d0c5 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -23,6 +23,7 @@
    + Argument `logicalAsInt` to `fwrite()` has been removed.
    + Argument `autostart` to `fread()` has been removed.
    + Argument `in.place` to `droplevels` has been removed.
+   + It's now an error to set `datatable.nomatch`, which has been warning since 1.15.0.
 
 # data.table [v1.17.0](https://github.com/Rdatatable/data.table/milestone/34)  (20 Feb 2025)
 
diff --git a/R/onLoad.R b/R/onLoad.R
index fea51fdc93..e3e9a783e7 100644
--- a/R/onLoad.R
+++ b/R/onLoad.R
@@ -1,15 +1,8 @@
 # nocov start
 
-.pkg.store = new.env()
-.pkg.store$.unsafe.done = FALSE
 .unsafe.opt = function() {
-  if (.pkg.store$.unsafe.done) return(invisible())
-  val = getOption("datatable.nomatch")
-  if (is.null(val)) return(invisible())  # not defined (it hasn't been defined in .onLoad since v1.12.4)
-  warningf("Option 'datatable.nomatch' is defined but is now ignored. Please see note 11 in v1.12.4 NEWS (Oct 2019), and note 14 in v1.14.2.")
-  # leave this as warning for a long time
-  .pkg.store$.unsafe.done = TRUE
-  invisible()
+  if (!is.null(getOption("datatable.nomatch")))
+    stopf("Option 'datatable.nomatch' is defined but is now ignored. Please see note 11 in v1.12.4 NEWS (Oct 2019), and note 14 in v1.14.2.")
 }
 
 .Last.updated = vector("integer", 1L) # exported variable; number of rows updated by the last := or set(), #1885