diff --git a/R/fmelt.R b/R/fmelt.R
index ba61e2b04..4c034641d 100644
--- a/R/fmelt.R
+++ b/R/fmelt.R
@@ -1,13 +1,15 @@
-melt.data.table <- function(data, id.vars = NULL, measure.vars = NULL, variable.name = "variable", 
+melt.data.table <- function(data, id.vars, measure.vars, variable.name = "variable", 
            value.name = "value", ..., na.rm = FALSE, variable.factor = TRUE, value.factor = FALSE, 
            verbose = getOption("datatable.verbose")) {
     drop.levels <- FALSE # maybe a future FR
     if (!inherits(data, "data.table")) stop("'data' must be a data.table")
-        ans <- .Call("Cfmelt", data, id.vars, measure.vars, 
-                as.logical(variable.factor), as.logical(value.factor), 
-                variable.name, value.name, 
-                as.logical(na.rm), as.logical(drop.levels), 
-                as.logical(verbose));
+    if (missing(id.vars)) id.vars=NULL
+    if (missing(measure.vars)) measure.vars = NULL
+    ans <- .Call("Cfmelt", data, id.vars, measure.vars, 
+            as.logical(variable.factor), as.logical(value.factor), 
+            variable.name, value.name, 
+            as.logical(na.rm), as.logical(drop.levels), 
+            as.logical(verbose));
     setattr(ans, "row.names", .set_row_names(length(ans[[1L]])))
     setattr(ans, "class", c("data.table", "data.frame"))
     alloc.col(ans)
diff --git a/README.md b/README.md
index aefd89703..62e9ae571 100644
--- a/README.md
+++ b/README.md
@@ -276,6 +276,8 @@ We moved from R-Forge to GitHub on 9 June 2014, including history.
 
   44. Subset on data.table using `lapply` of the form `lapply(L, "[", Time == 3L)` works now without error due to "[.data.frame" redirection. Closes [#500](https://github.com/Rdatatable/data.table/issues/500). Thanks to Garrett See for reporting.
 
+  45. `id.vars` and `measure.vars` default value of `NULL` was removed to be consistent in behaviour with `reshape2:::melt.data.frame`. Closes [#780](https://github.com/Rdatatable/data.table/issues/500). Thanks to @dardesta for reporting.
+
 #### NOTES
 
   1.  Reminder: using `rolltolast` still works but since v1.9.2 now issues the following warning:  
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index b0f90c10b..f65309c4e 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -5110,6 +5110,17 @@ for (i in seq_along(dt)) {
     }) 
 }
 
+# fix for #780.
+if ("package:reshape2" %in% search()) {
+    try(detach(package:reshape),silent=TRUE)
+    DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
+    foo = function(input, by, var) {
+        melt(input, id.vars = by, measure.vars=var)
+    }
+    test(1371.1, foo(DT, by="x"), data.table(x=rep(DT$x, 2L), variable=factor(rep(c("y", "v"), each=9L), levels=c("y", "v")), value=c(DT$y, DT$v)), warning="All 'measure.vars are NOT of the SAME type.")
+    test(1371.2, foo(DT), data.table(x=rep(DT$x, 2L), variable=factor(rep(c("y", "v"), each=9L), levels=c("y", "v")), value=c(DT$y, DT$v)), warning="To be consistent with reshape2's melt, id.vars and")
+}
+
 ##########################
 
 
diff --git a/man/melt.data.table.Rd b/man/melt.data.table.Rd
index ce4141eed..9c54e94da 100644
--- a/man/melt.data.table.Rd
+++ b/man/melt.data.table.Rd
@@ -7,7 +7,7 @@
 }
 \usage{
 ## fast melt a data.table
-\method{melt}{data.table}(data, id.vars = NULL, measure.vars = NULL, 
+\method{melt}{data.table}(data, id.vars, measure.vars, 
 	variable.name = "variable", value.name = "value", 
 	..., na.rm = FALSE, variable.factor = TRUE, 
 	value.factor = FALSE, 
@@ -15,8 +15,8 @@
 }
 \arguments{
   \item{data}{ A \code{data.table} object to melt.}
-  \item{id.vars}{vector of id variables. Can be integer (corresponding id column numbers) or character (id column names) vector. If \code{NULL}, all non-measure columns will be assigned to it.}
-  \item{measure.vars}{vector of measure variables. Can be integer (corresponding measue column numbers) or character (measure column names) vector. If \code{NULL}, all non-id columns will be assigned to it.}
+  \item{id.vars}{vector of id variables. Can be integer (corresponding id column numbers) or character (id column names) vector. If missing, all non-measure columns will be assigned to it.}
+  \item{measure.vars}{vector of measure variables. Can be integer (corresponding measue column numbers) or character (measure column names) vector. If missing, all non-id columns will be assigned to it.}
   \item{variable.name}{name for the measured variable names column. The default name is 'variable'.}
   \item{value.name}{name for the molten data values column. The default name is 'value'.}
   \item{na.rm}{If \code{TRUE}, \code{NA} values will be removed from the molten data.}
@@ -26,7 +26,7 @@
   \item{...}{any other arguments to be passed to/from other methods}
 }
 \details{
-If \code{id.vars} and \code{measure.vars} are both \code{NULL}, all non-\code{numeric/integer/logical} columns are assigned as id variables and the rest of the columns are assigned as measure variables. If only one of \code{id.vars} or \code{measure.vars} is supplied, the rest of the columns will be assigned to the other. Both \code{id.vars} and \code{measure.vars} can have the same column more than once and same column can be as id and measure variables. 
+If \code{id.vars} and \code{measure.vars} are both missing, all non-\code{numeric/integer/logical} columns are assigned as id variables and the rest of the columns are assigned as measure variables. If only one of \code{id.vars} or \code{measure.vars} is supplied, the rest of the columns will be assigned to the other. Both \code{id.vars} and \code{measure.vars} can have the same column more than once and same column can be as id and measure variables. 
 
 \code{melt.data.table} also accepts \code{list} columns for both id and measure variables. When all \code{measure.vars} are not of the same type, they'll be coerced according to the hierarchy \code{list} > \code{character} > \code{numeric > integer > logical}. For example, any of the measure variables is a \code{list}, then entire value column will be coerced to a list. Note that, if the type of \code{value} column is a list, \code{na.rm = TRUE} will have no effect.