diff --git a/R/between.R b/R/between.R
index e96434732..ab968b6eb 100644
--- a/R/between.R
+++ b/R/between.R
@@ -76,8 +76,10 @@ inrange = function(x,lower,upper,incbounds=TRUE) {
   verbose = isTRUE(getOption("datatable.verbose"))
   if (verbose) {last.started.at=proc.time();catf("forderv(query) took ... ");flush.console()}
   if (verbose) {cat(timetaken(last.started.at),"\n"); flush.console()}
-  ans = bmerge(shallow(subject), query, 1L:2L, c(1L,1L),
-      0, c(FALSE, TRUE), 0L, "all", ops, verbose) # fix for #1819, turn on verbose messages
+  ans = bmerge(
+    shallow(subject), query,
+    icols=1L:2L, xcols=c(1L,1L),
+    roll=0.0, rollends=c(FALSE, TRUE), nomatch=0L, mult="all", ops, verbose) # fix for #1819, turn on verbose messages
   xo = ans$xo
   options(datatable.verbose=FALSE)
   setDT(ans[c("starts", "lens")], key=c("starts", "lens"))
diff --git a/R/bmerge.R b/R/bmerge.R
index f81461574..ef7790f27 100644
--- a/R/bmerge.R
+++ b/R/bmerge.R
@@ -22,6 +22,13 @@ coerce_col = function(dt, col, from_type, to_type, from_name, to_name, verbose_m
 
 bmerge = function(i, x, icols, xcols, roll, rollends, nomatch, mult, ops, verbose)
 {
+  if (roll != 0.0 && length(icols)) {
+    last_x_idx = tail(xcols, 1L)
+    last_i_idx = tail(icols, 1L)
+    if (is.factor(x[[last_x_idx]]) || is.factor(i[[last_i_idx]]))
+      stopf("Attempting roll join on factor column when joining x.%s to i.%s. Only integer, double or character columns may be roll joined.", names(x)[last_x_idx], names(i)[last_i_idx])
+  }
+
   callersi = i
   i = shallow(i)
   # Just before the call to bmerge() in [.data.table there is a shallow() copy of i to prevent coercions here
@@ -59,9 +66,8 @@ bmerge = function(i, x, icols, xcols, roll, rollends, nomatch, mult, ops, verbos
     iname = paste0("i.", names(i)[icol])
     if (!x_merge_type %chin% supported) stopf("%s is type %s which is not supported by data.table join", xname, x_merge_type)
     if (!i_merge_type %chin% supported) stopf("%s is type %s which is not supported by data.table join", iname, i_merge_type)
+    # we check factors first because they might have different levels
     if (x_merge_type=="factor" || i_merge_type=="factor") {
-      if (roll!=0.0 && a==length(icols))
-        stopf("Attempting roll join on factor column when joining %s to %s. Only integer, double or character columns may be roll joined.", xname, iname)
       if (x_merge_type=="factor" && i_merge_type=="factor") {
         if (verbose) catf("Matching %s factor levels to %s factor levels.\n", iname, xname)
         set(i, j=icol, value=chmatch(levels(i[[icol]]), levels(x[[xcol]]), nomatch=0L)[i[[icol]]])  # nomatch=0L otherwise a level that is missing would match to NA values
@@ -82,7 +88,6 @@ bmerge = function(i, x, icols, xcols, roll, rollends, nomatch, mult, ops, verbos
       }
       stopf("Incompatible join types: %s (%s) and %s (%s). Factor columns must join to factor or character columns.", xname, x_merge_type, iname, i_merge_type)
     }
-    # we check factors first to cater for the case when trying to do rolling joins on factors
     if (x_merge_type == i_merge_type) {
       if (verbose) catf("%s has same type (%s) as %s. No coercion needed.\n", iname, x_merge_type, xname)
       next