diff --git a/NEWS.md b/NEWS.md
index 92d818186..5f825c023 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -8,6 +8,8 @@
 
 1. `hasNA` argument to `froll*` functions no longer works. Use `has.nf` instead; see the release note for 1.18.0, item 3 under `BREAKING CHANGE`.
 
+2. `x` and `n` arguments to `frollapply` functions no longer work. Use `X` and `N` instead, respectively; see the release note for 1.18.0, item 4 under `BREAKING CHANGE`.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).
diff --git a/R/frollapply.R b/R/frollapply.R
index 80bc65f7c..a8a214ff1 100644
--- a/R/frollapply.R
+++ b/R/frollapply.R
@@ -127,12 +127,10 @@ fixselfref = function(x) {
 
 frollapply = function(X, N, FUN, ..., by.column=TRUE, fill=NA, align=c("right","left","center"), adaptive=FALSE, partial=FALSE, give.names=FALSE, simplify=TRUE, x, n) {
   if (!missing(x)) {
-    warningf("'x' is deprecated in frollapply, use 'X' instead")
-    X = x
+    stopf("'x' is deprecated in frollapply, use 'X' instead")
   }
   if (!missing(n)) {
-    warningf("'n' is deprecated in frollapply, use 'N' instead")
-    N = n
+    stopf("'n' is deprecated in frollapply, use 'N' instead")
   }
   if (!isTRUEorFALSE(by.column))
     stopf("'by.column' must be TRUE or FALSE")
diff --git a/inst/tests/froll.Rraw b/inst/tests/froll.Rraw
index b41f4ce99..3171c16a2 100644
--- a/inst/tests/froll.Rraw
+++ b/inst/tests/froll.Rraw
@@ -1944,9 +1944,9 @@ test(6010.542, frollapply(1:2, 2, sum, adaptive=NA), error="must be TRUE or FALS
 test(6010.543, frollapply(1:2, 2, sum, partial=NA), error="must be TRUE or FALSE")
 test(6010.544, frollapply(1:2, 2, sum, give.names=NA), error="must be TRUE or FALSE")
 test(6010.545, frollapply(1:2, 2, sum, simplify=NA), error="must be TRUE or FALSE or a function")
-test(6010.561, frollapply(x=1:2, N=2, FUN=sum), c(NA,3L), warning="'x' is deprecated in frollapply, use 'X' instead")
-test(6010.562, frollapply(X=1:2, n=2, FUN=sum), c(NA,3L), warning="'n' is deprecated in frollapply, use 'N' instead")
-test(6010.563, frollapply(x=1:2, n=2, FUN=sum), c(NA,3L), warning=c("'x' is deprecated in frollapply, use 'X' instead","'n' is deprecated in frollapply, use 'N' instead"))
+test(6010.561, frollapply(x=1:2, N=2, FUN=sum), error="'x' is deprecated in frollapply, use 'X' instead")
+test(6010.562, frollapply(X=1:2, n=2, FUN=sum), error="'n' is deprecated in frollapply, use 'N' instead")
+# 6010.563 tested both x= and n= resulting in 2 warnings, but with errors, that's not very useful.
 test(6010.564, frollapply(1:2, c("a","a"), length, adaptive=TRUE), error="'N' must be an integer vector or list of integer vectors")
 test(6010.565, frollapply(1:2, list(c("a","a")), length, adaptive=TRUE), error="'N' must be an integer vector or list of integer vectors")
 test(6010.566, frollapply(1:2, 2, length, by.column=FALSE), error="frollapply by.column=FALSE requires 'X' argument to be")