diff --git a/R/cmtk_io.R b/R/cmtk_io.R
index 74074996..c7949987 100644
--- a/R/cmtk_io.R
+++ b/R/cmtk_io.R
@@ -154,9 +154,9 @@ read.cmtk<-function(con, CheckLabel=TRUE){
 }
 
 #' Write out CMTK registration list to folder
-#' 
+#'
 #' @details Note that transformation in the forward direction (i.e. sample->ref)
-#'   e.g. as calculated from a set of landmarks where set 1 is the sample is 
+#'   e.g. as calculated from a set of landmarks where set 1 is the sample is
 #'   considered an inverse transformation by the IGS software. So in order to
 #'   use such a transformation as an initial affine with the registration
 #'   command the switch --initial-inverse must be used specifying the folder
@@ -168,11 +168,16 @@ read.cmtk<-function(con, CheckLabel=TRUE){
 #'   by default.
 #' @param reglist List specifying CMTK registration parameters
 #' @param foldername Path to registration folder (usually ending in .list)
-#' @param version CMTK version for registration (default 2.4)
+#' @param version CMTK version for registration (default 2.4). Will be converted
+#'   to character vector if not already.
 #' @export
 #' @family cmtk-io
-write.cmtkreg<-function(reglist, foldername, version="2.4"){
-  if(!is.null(attr(reglist, 'version')) && (attr(reglist, 'version') != version)) warning("Specified version (", version, ") is not the same as the version stored in the reglist object (", attr(reglist, 'version'), ").")
+write.cmtkreg<-function(reglist, foldername, version="2.4") {
+  if(!is.character(version))
+    version=as.character(version)
+  if(!is.null(attr(reglist, 'version')) && (attr(reglist, 'version') != version))
+    warning("Specified version (", version, ") is not the same as the version stored in the reglist object (", attr(reglist, 'version'), ").")
+  
   dir.create(foldername, showWarnings=FALSE, recursive=TRUE)
   if(!is.list(reglist)) reglist=cmtkreglist(reglist)
   write.cmtk(reglist,file.path(foldername, "registration"),
diff --git a/man/write.cmtkreg.Rd b/man/write.cmtkreg.Rd
index 3851e896..97e7e31e 100644
--- a/man/write.cmtkreg.Rd
+++ b/man/write.cmtkreg.Rd
@@ -11,14 +11,15 @@ write.cmtkreg(reglist, foldername, version = "2.4")
 
 \item{foldername}{Path to registration folder (usually ending in .list)}
 
-\item{version}{CMTK version for registration (default 2.4)}
+\item{version}{CMTK version for registration (default 2.4). Will be converted
+to character vector if not already.}
 }
 \description{
 Write out CMTK registration list to folder
 }
 \details{
 Note that transformation in the forward direction (i.e. sample->ref)
-  e.g. as calculated from a set of landmarks where set 1 is the sample is 
+  e.g. as calculated from a set of landmarks where set 1 is the sample is
   considered an inverse transformation by the IGS software. So in order to
   use such a transformation as an initial affine with the registration
   command the switch --initial-inverse must be used specifying the folder
diff --git a/tests/testthat/test-cmtk_io.R b/tests/testthat/test-cmtk_io.R
index 64250b70..a88a2196 100644
--- a/tests/testthat/test-cmtk_io.R
+++ b/tests/testthat/test-cmtk_io.R
@@ -19,7 +19,7 @@ test_that("read.cmtk and write.cmtk can round-trip a registration file", {
   reglist=read.cmtkreg(reg)
   tf=tempfile('dofv1.1wshears_copy',fileext='.list')
   on.exit(unlink(tf,recursive=TRUE))
-  write.cmtkreg(reglist,foldername=tf,version=1.1)
+  write.cmtkreg(reglist, foldername=tf, version=1.1)
   ctf=cmtkreg(tf,returnDir=TRUE)
   # equivalent because we are not interested in the file.info attributes
   # though this also removes version attribute which might be worth a thought