diff --git a/0001-Fix-Bytes-String-for-OCaml-4.06.patch b/0001-Fix-Bytes-String-for-OCaml-4.06.patch
new file mode 100644
index 0000000..4d1408a
--- /dev/null
+++ b/0001-Fix-Bytes-String-for-OCaml-4.06.patch
@@ -0,0 +1,58 @@
+From 5c5eff66dfaccb212b8906e769e40633d8b8f5e4 Mon Sep 17 00:00:00 2001
+From: "Richard W.M. Jones" <rjones@redhat.com>
+Date: Tue, 13 Feb 2018 08:20:52 +0000
+Subject: [PATCH] Fix Bytes/String for OCaml 4.06.
+
+---
+ src/format_ext2_kernel.ml |  4 ++--
+ src/mode_build.ml         | 10 ++++++----
+ 2 files changed, 8 insertions(+), 6 deletions(-)
+
+diff --git a/src/format_ext2_kernel.ml b/src/format_ext2_kernel.ml
+index d5d529d..98bff3a 100644
+--- a/src/format_ext2_kernel.ml
++++ b/src/format_ext2_kernel.ml
+@@ -290,9 +290,9 @@ and read_leshort chan offset =
+ 
+ and read_string chan offset len =
+   seek_in chan offset;
+-  let buf = String.create len in
++  let buf = Bytes.create len in
+   really_input chan buf 0 len;
+-  buf
++  Bytes.to_string buf
+ 
+ and copy_or_symlink_file copy_kernel src dest =
+   if not copy_kernel then
+diff --git a/src/mode_build.ml b/src/mode_build.ml
+index 95869cb..b5f5fa6 100644
+--- a/src/mode_build.ml
++++ b/src/mode_build.ml
+@@ -299,9 +299,10 @@ and update_appliance appliance lines = function
+ (* Determine the [file_type] of [file], or exit with an error. *)
+ and get_file_type file =
+   let chan = open_in file in
+-  let buf = String.create 512 in
+-  let len = input chan buf 0 (String.length buf) in
++  let buf = Bytes.create 512 in
++  let len = input chan buf 0 (Bytes.length buf) in
+   close_in chan;
++  let buf = Bytes.to_string buf in
+ 
+   if len >= 3 && buf.[0] = '\x1f' && buf.[1] = '\x8b' && buf.[2] = '\x08'
+   then                                  (* gzip-compressed file *)
+@@ -335,8 +336,9 @@ and get_file_content file buf len =
+ and get_compressed_file_content zcat file =
+   let cmd = sprintf "%s %s" zcat (quote file) in
+   let chan_out, chan_in, chan_err = open_process_full cmd [||] in
+-  let buf = String.create 512 in
+-  let len = input chan_out buf 0 (String.length buf) in
++  let buf = Bytes.create 512 in
++  let len = input chan_out buf 0 (Bytes.length buf) in
++  let buf = Bytes.to_string buf in
+   (* We're expecting the subprocess to fail because we close the pipe
+    * early, so:
+    *)
+-- 
+2.15.1
+
diff --git a/supermin.spec b/supermin.spec
index a34e85e..c4dec25 100644
--- a/supermin.spec
+++ b/supermin.spec
@@ -31,7 +31,7 @@
 Summary:       Tool for creating supermin appliances
 Name:          supermin
 Version:       5.1.19
-Release:       2%{?dist}
+Release:       3%{?dist}
 License:       GPLv2+
 
 %if 0%{?rhel} >= 7
@@ -49,6 +49,9 @@ Source1:       http://libguestfs.org/download/supermin/%{name}-%{version}.tar.gz
 Source2:       libguestfs.keyring
 %endif
 
+# Upstream patch which fixes byte/string problems.
+Patch1:        0001-Fix-Bytes-String-for-OCaml-4.06.patch
+
 BuildRequires: /usr/bin/pod2man
 BuildRequires: /usr/bin/pod2html
 BuildRequires: rpm
@@ -170,6 +173,9 @@ make check || {
 
 
 %changelog
+* Tue Feb 13 2018 Richard W.M. Jones <rjones@redhat.com> - 5.1.19-3
+- Fix bytes/string problems.
+
 * Fri Feb 09 2018 Fedora Release Engineering <releng@fedoraproject.org> - 5.1.19-2
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_28_Mass_Rebuild