diff --git a/R/data.table.R b/R/data.table.R
index fe0f42a56..7b48704a1 100644
--- a/R/data.table.R
+++ b/R/data.table.R
@@ -199,7 +199,7 @@ replace_dot_alias = function(e) {
     }
     return(x)
   }
-  if (!mult %chin% c("first", "last", "all")) stopf("mult argument can only be 'first', 'last' or 'all'")
+  if (!mult %chin% c("first","last","all")) stopf("mult argument can only be 'first', 'last' or 'all'")
   missingroll = missing(roll)
   if (length(roll)!=1L || is.na(roll)) stopf("roll must be a single TRUE, FALSE, positive/negative integer/double including +Inf and -Inf or 'nearest'")
   if (is.character(roll)) {
diff --git a/src/bmerge.c b/src/bmerge.c
index b3e993e19..f0e55dc02 100644
--- a/src/bmerge.c
+++ b/src/bmerge.c
@@ -29,7 +29,7 @@ static SEXP nqgrp;
 static int ncol, *o, *xo, *retFirst, *retLength, *retIndex, *allLen1, *allGrp1, *rollends, ilen, anslen;
 static int *op, nqmaxgrp;
 static int ctr, nomatch; // populating matches for non-equi joins
-enum {ALL, FIRST, LAST} mult = ALL;
+enum {ALL, FIRST, LAST, ERR} mult = ALL;
 static double roll, rollabs;
 static Rboolean rollToNearest=FALSE;
 #define XIND(i) (xo ? xo[(i)]-1 : i)
@@ -113,6 +113,8 @@ SEXP bmerge(SEXP idt, SEXP xdt, SEXP icolsArg, SEXP xcolsArg, SEXP xoArg, SEXP r
     mult = FIRST;
   else if (!strcmp(CHAR(STRING_ELT(multArg, 0)), "last"))
     mult = LAST;
+  else if (!strcmp(CHAR(STRING_ELT(multArg, 0)), "error"))
+    mult = ERR;
   else
     internal_error(__func__, "invalid value for 'mult'"); // # nocov
 
@@ -426,7 +428,7 @@ void bmerge_r(int xlowIn, int xuppIn, int ilowIn, int iuppIn, int col, int thisg
         for (int j=ilow+1; j<iupp; j++) {
           const int k = o ? o[j]-1 : j;
           if (retFirst[k] != nomatch) {
-            if (mult == ALL) {
+            if (mult == ALL || mult == ERR) { // len>1 && mult==ERR already checked, no dup matches, continue as mult=ALL
               // for this irow, we've matches on more than one group
               allGrp1[0] = FALSE;
               retFirst[ctr+ilen] = xlow+2;
@@ -448,7 +450,7 @@ void bmerge_r(int xlowIn, int xuppIn, int ilowIn, int iuppIn, int col, int thisg
             }
           } else {
             // none of the groups so far have filled in for this index. So use it!
-            if (mult == ALL) {
+            if (mult == ALL || mult == ERR) {
               retFirst[k] = xlow+2;
               retLength[k] = len;
               retIndex[k] = k+1;