diff --git a/pkgs/tools/filesystems/mtools/default.nix b/pkgs/tools/filesystems/mtools/default.nix
index 762368651d70..1cbe48ecee54 100644
--- a/pkgs/tools/filesystems/mtools/default.nix
+++ b/pkgs/tools/filesystems/mtools/default.nix
@@ -1,16 +1,17 @@
 { stdenv, fetchurl }:
 
 stdenv.mkDerivation rec {
-  name = "mtools-4.0.19";
+  name = "mtools-4.0.20";
 
   src = fetchurl {
     url = "mirror://gnu/mtools/${name}.tar.bz2";
-    sha256 = "1pqhv5l4fqj1d3698vajkccz65xqxs3991wpylbw7hm1kqcrgh8v";
+    sha256 = "1vcahr9s6zv1hnrx2bgjnzcas2y951q90r1jvvv4q9v5kwfd6qb0";
   };
 
   # Prevents errors such as "mainloop.c:89:15: error: expected ')'"
   # Upstream issue https://lists.gnu.org/archive/html/info-mtools/2014-02/msg00000.html
-  patches = stdenv.lib.optional stdenv.isDarwin ./UNUSED-darwin.patch;
+  patches = [ ./fix-dos_to_wchar-declaration.patch ] ++
+    stdenv.lib.optional stdenv.isDarwin ./UNUSED-darwin.patch;
 
   # fails to find X on darwin
   configureFlags = stdenv.lib.optional stdenv.isDarwin "--without-x";
diff --git a/pkgs/tools/filesystems/mtools/fix-dos_to_wchar-declaration.patch b/pkgs/tools/filesystems/mtools/fix-dos_to_wchar-declaration.patch
new file mode 100644
index 000000000000..576a14b1cb46
--- /dev/null
+++ b/pkgs/tools/filesystems/mtools/fix-dos_to_wchar-declaration.patch
@@ -0,0 +1,11 @@
+--- mtools-4.0.20.org/charsetConv.c	2018-11-19 10:16:14.183820865 +0000
++++ mtools-4.0.20/charsetConv.c	2018-11-19 10:15:39.808451465 +0000
+@@ -266,7 +266,7 @@
+ 	free(cp);
+ }
+ 
+-int dos_to_wchar(doscp_t *cp, char *dos, wchar_t *wchar, size_t len)
++int dos_to_wchar(doscp_t *cp, const char *dos, wchar_t *wchar, size_t len)
+ {
+ 	int i;
+