diff --git a/mono-configure-c99.patch b/mono-configure-c99.patch
new file mode 100644
index 0000000..0f78563
--- /dev/null
+++ b/mono-configure-c99.patch
@@ -0,0 +1,76 @@
+The thread start routine must return void *, and int and void *
+are distinct types.  Compilers increasingly issue errors instead
+of warnings for such type errors, and this causes the configure
+probe to fail unconditionally, even if the system supports
+__thread variables.
+
+Submitted upstream: <https://github.com/mono/mono/pull/21730>
+
+diff --git a/configure.ac b/configure.ac
+index bbbd1622d9728859..5a3a5770c8a3cdb6 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -2833,14 +2833,16 @@ if test x$host_win32 = xno; then
+ 			__thread int i;
+ 			static int res1, res2;
+ 
+-			void thread_main (void *arg)
++			void *thread_main (void *parg)
+ 			{
++				int arg = *(int *)parg;
+ 				i = arg;
+ 				sleep (1);
+ 				if (arg == 1)
+ 					res1 = (i == arg);
+ 				else
+ 					res2 = (i == arg);
++				return NULL;
+ 			}
+ 
+ 			int main () {
+@@ -2848,8 +2850,10 @@ if test x$host_win32 = xno; then
+ 
+ 				i = 5;
+ 
+-				pthread_create (&t1, NULL, thread_main, 1);
+-				pthread_create (&t2, NULL, thread_main, 2);
++				int one = 1;
++				pthread_create (&t1, NULL, thread_main, &one);
++				int two = 2;
++				pthread_create (&t2, NULL, thread_main, &two);
+ 
+ 				pthread_join (t1, NULL);
+ 				pthread_join (t2, NULL);
+--- a/configure	2023-06-14 17:18:48.000000000 +0200
++++ b/configure	2023-12-09 10:41:58.001084368 +0100
+@@ -24906,14 +24894,16 @@
+ 			__thread int i;
+ 			static int res1, res2;
+ 
+-			void thread_main (void *arg)
++			void *thread_main (void *parg)
+ 			{
++				int arg = *(int *)parg;
+ 				i = arg;
+ 				sleep (1);
+ 				if (arg == 1)
+ 					res1 = (i == arg);
+ 				else
+ 					res2 = (i == arg);
++				return NULL;
+ 			}
+ 
+ 			int main () {
+@@ -24921,8 +24911,10 @@
+ 
+ 				i = 5;
+ 
+-				pthread_create (&t1, NULL, thread_main, 1);
+-				pthread_create (&t2, NULL, thread_main, 2);
++				int one = 1;
++				pthread_create (&t1, NULL, thread_main, &one);
++				int two = 2;
++				pthread_create (&t2, NULL, thread_main, &two);
+ 
+ 				pthread_join (t1, NULL);
+ 				pthread_join (t2, NULL);
diff --git a/mono.spec b/mono.spec
index 82603fd..0059800 100644
--- a/mono.spec
+++ b/mono.spec
@@ -24,7 +24,7 @@
 %global xamarinrelease 199
 Name:           mono
 Version:        6.12.0
-Release:        13%{?dist}
+Release:        14%{?dist}
 Summary:        Cross-platform, Open Source, .NET development framework
 
 License:        MIT
@@ -58,6 +58,7 @@ Patch11:        0001-Replace-new-Csharp-features-with-old-ones.patch
 Patch12:        0001-Reenable-mdoc.exe-build.patch
 # fix issue with conflicts between i686 and x86_64 package (#1853724)
 Patch13:	mono-6.6.0-fix-multi-arch-issue.patch
+Patch14:        mono-configure-c99.patch
 
 BuildRequires: make
 BuildRequires:  bison
@@ -349,6 +350,7 @@ pushd external/api-doc-tools
 popd
 %patch -P 12 -p1
 %patch -P 13 -p1
+%patch -P 14 -p1
 
 # don't build mono-helix-client which requires the helix-binaries to build
 sed -i 's|mono-helix-client||g' mcs/tools/Makefile
@@ -941,6 +943,9 @@ cert-sync --quiet /etc/pki/tls/certs/ca-bundle.crt
 %files complete
 
 %changelog
+* Sat Dec  9 2023 Florian Weimer <fweimer@redhat.com> - 6.12.0-14
+- Fix C compatibility issue in the configure script
+
 * Sat Aug 12 2023 Timotheus Pokorra <timotheus.pokorra@solidcharity.com> - 6.12.0-13
 - Upgrade to Mono 6.12.0.199