diff --git a/R/fcast.R b/R/fcast.R
index c0dbfd4e3a..7b633823f7 100644
--- a/R/fcast.R
+++ b/R/fcast.R
@@ -136,6 +136,12 @@ dcast.data.table = function(data, formula, fun.aggregate = NULL, sep = "_", ...,
     value.var = names(data)[ncol(data)]
   lvals = value_vars(value.var, names(data))
   valnames = unique(unlist(lvals))
+  if (any(valnames == "")) {
+    valnames[valnames == ""] = ""
+  }
+  if (any(duplicated(valnames))) {
+    valnames = make.unique(valnames, sep = sep)
+  }
   lvars = check_formula(formula, names(data), valnames, value.var.in.LHSdots, value.var.in.RHSdots)
   lvars = lapply(lvars, function(x) if (length(x)) x else quote(`.`))
   # tired of lapply and the way it handles environments!
@@ -252,4 +258,4 @@ dcast.data.table = function(data, formula, fun.aggregate = NULL, sep = "_", ...,
     setDT(ans); setattr(ans, 'sorted', lhsnames)
   } else internal_error("empty rhsnames") # nocov
   return(ans)
-}
+}
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