diff --git a/mono-5.18.0-largearraybuilder.patch b/mono-5.18.0-largearraybuilder.patch
new file mode 100644
index 0000000..0df61a9
--- /dev/null
+++ b/mono-5.18.0-largearraybuilder.patch
@@ -0,0 +1,41 @@
+From 0bf46dbe2cf0a215ca6e64793b7c434433f50722 Mon Sep 17 00:00:00 2001
+From: Marek Safar <marek.safar@gmail.com>
+Date: Wed, 31 Oct 2018 10:25:58 +0100
+Subject: [PATCH] Fixes __MonoCS__ handling of value types
+
+---
+ .../src/System/Collections/Generic/LargeArrayBuilder.cs     | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/external/corefx/src/Common/src/System/Collections/Generic/LargeArrayBuilder.cs b/external/corefx/src/Common/src/System/Collections/Generic/LargeArrayBuilder.cs
+index c0f441909dcf..8ec603ace72e 100644
+--- a/external/corefx/src/Common/src/System/Collections/Generic/LargeArrayBuilder.cs
++++ b/external/corefx/src/Common/src/System/Collections/Generic/LargeArrayBuilder.cs
+@@ -263,7 +263,7 @@ public CopyPosition CopyTo(CopyPosition position, T[] array, int arrayIndex, int
+             T[] buffer = GetBuffer(row);
+             int copied =
+ #if __MonoCS__
+-            CopyToCore(buffer, column, array, arrayIndex, count);
++            CopyToCore(buffer, column, array, ref arrayIndex, ref count);
+ #else
+             CopyToCore(buffer, column);
+ #endif
+@@ -278,7 +278,7 @@ public CopyPosition CopyTo(CopyPosition position, T[] array, int arrayIndex, int
+                 buffer = GetBuffer(++row);
+                 copied =
+ #if __MonoCS__
+-                CopyToCore(buffer, 0, array, arrayIndex, count);
++                CopyToCore(buffer, 0, array, ref arrayIndex, ref count);
+ #else
+                 CopyToCore(buffer, 0);
+ #endif
+@@ -289,7 +289,7 @@ public CopyPosition CopyTo(CopyPosition position, T[] array, int arrayIndex, int
+ #if __MonoCS__
+         }
+ 
+-        static int CopyToCore(T[] sourceBuffer, int sourceIndex, T[] array, int arrayIndex, int count)
++        static int CopyToCore(T[] sourceBuffer, int sourceIndex, T[] array, ref int arrayIndex, ref int count)
+ #else
+             int CopyToCore(T[] sourceBuffer, int sourceIndex)
+ #endif
+
diff --git a/mono.spec b/mono.spec
index 4ba622c..20ab256 100644
--- a/mono.spec
+++ b/mono.spec
@@ -23,7 +23,7 @@
 %global xamarinrelease 3
 Name:           mono
 Version:        5.18.1
-Release:        5%{?dist}
+Release:        6%{?dist}
 Summary:        Cross-platform, Open Source, .NET development framework
 
 License:        MIT
@@ -47,6 +47,7 @@ Patch9:         mono-5.18.0-reference-assemblies-fix.patch
 Patch10:        mono-5.18.0-sharpziplib-parent-path-traversal.patch
 Patch11:        mono-5.18.1-python3.patch
 Patch12:        mono-5.18.1-s390x-build.patch
+Patch13:        mono-5.18.0-largearraybuilder.patch
 
 BuildRequires:  bison
 BuildRequires:  python%{python3_pkgversion}
@@ -336,6 +337,7 @@ not install anything from outside the mono source (XSP, mono-basic, etc.).
 %patch10 -p1
 %patch11 -p1
 %patch12 -p1
+%patch13 -p1
 
 # Remove hardcoded lib directory for libMonoPosixHelper.so from the config
 sed -i 's|$mono_libdir/||g' data/config.in
@@ -883,6 +885,9 @@ cert-sync /etc/pki/tls/certs/ca-bundle.crt
 %files complete
 
 %changelog
+* Wed May 01 2019 Timotheus Pokorra <timotheus.pokorra@solidcharity.com> - 5.18.1-6
+- backport a fix for LargeArrayBuilder, fixes bug 1704847
+
 * Wed May 01 2019 Timotheus Pokorra <timotheus.pokorra@solidcharity.com> - 5.18.1-5
 - mono-devel should not provide for namespaces in the reference assemblies. fixes bug 1704560