diff --git a/runtime/COOP.md b/runtime/COOP.md
index a2501c4f00b1..086ef1fd67a6 100644
--- a/runtime/COOP.md
+++ b/runtime/COOP.md
@@ -120,3 +120,41 @@ Debugging tips
 * Display the current thread state:
 
     display (void *) mono_thread_info_current ()->thread_state
+
+Enable GC Assertions
+====================
+
+Apply this patch to the iOS SDK (here only done for the `arm64_32` runtime):
+```patch
+diff --git a/sdks/builds/ios.mk b/sdks/builds/ios.mk
+index 571dbd797a8..5bbe30040d2 100644
+--- a/sdks/builds/ios.mk
++++ b/sdks/builds/ios.mk
+@@ -155,7 +155,7 @@ watchos64_32_sysroot = -isysroot $(watchos64_32_sysroot_path)
+ # explicitly disable dtrace, since it requires inline assembly, which is disabled on AppleTV (and mono's configure.ac doesn't know that (yet at least))
+ ios-targettv_CONFIGURE_FLAGS =         --enable-dtrace=no --enable-llvm-runtime --with-bitcode=yes
+ ios-targetwatch_CONFIGURE_FLAGS = --enable-cooperative-suspend --enable-llvm-runtime --with-bitcode=yes
+-ios-targetwatch64_32_CONFIGURE_FLAGS = --enable-cooperative-suspend --enable-llvm-runtime --with-bitcode=yes
++ios-targetwatch64_32_CONFIGURE_FLAGS = --enable-cooperative-suspend --enable-llvm-runtime --with-bitcode=yes --enable-checked-build=gc
+
+ ios-target32_SYSROOT = $(ios_sysroot) -miphoneos-version-min=$(IOS_VERSION_MIN)
+ ios-target32s_SYSROOT = $(ios_sysroot) -miphoneos-version-min=$(IOS_VERSION_MIN)
+```
+
+and make sure you build `xamarin-macios` with `MONO_BUILD_FROM_SOURCE=1`. Also modify `mtouch` such that it enables checked-build:
+
+```patch
+diff --git a/tools/mtouch/mtouch.cs b/tools/mtouch/mtouch.cs
+index 1ef159b79..cb2caf426 100644
+--- a/tools/mtouch/mtouch.cs
++++ b/tools/mtouch/mtouch.cs
+@@ -649,6 +654,8 @@ namespace Xamarin.Bundler
+                                                sw.WriteLine ("\tsetenv (\"MONO_GC_PARAMS\", \"{0}\", 1);", app.MonoGCParams);
+                                        foreach (var kvp in app.EnvironmentVariables)
+                                                sw.WriteLine ("\tsetenv (\"{0}\", \"{1}\", 1);", kvp.Key.Replace ("\"", "\\\""), kvp.Value.Replace ("\"", "\\\""));
++                                       sw.WriteLine ("\tsetenv (\"MONO_CHECK_MODE\", \"gc\", 1);");
++
+                                        sw.WriteLine ("\txamarin_supports_dynamic_registration = {0};", app.DynamicRegistrationSupported ? "TRUE" : "FALSE");
+                                        sw.WriteLine ("}");
+                                        sw.WriteLine ();
+```
diff --git a/runtime/runtime.m b/runtime/runtime.m
index 19e9e74bfcaf..79cdc638aadf 100644
--- a/runtime/runtime.m
+++ b/runtime/runtime.m
@@ -512,7 +512,7 @@ -(void) xamarinSetGCHandle: (int) gc_handle;
 get_raw_gchandle_safe (id self)
 {
 	// COOP: we call a selector, and that must only be done in SAFE mode.
-	MONO_ASSERT_GC_SAFE;
+	MONO_ASSERT_GC_SAFE_OR_DETACHED;
 	id<XamarinExtendedObject> xself = self;
 	return (int) [xself xamarinGetGCHandle];
 }
@@ -2361,8 +2361,10 @@ -(void) xamarinSetGCHandle: (int) gc_handle;
 		exc_handle = [[ns_exception userInfo] objectForKey: @"XamarinManagedExceptionHandle"];
 		if (exc_handle != NULL) {
 			int handle = [exc_handle getHandle];
+			MONO_ENTER_GC_UNSAFE;
 			MonoObject *exc = mono_gchandle_get_target (handle);
 			mono_runtime_set_pending_exception ((MonoException *) exc, false);
+			MONO_EXIT_GC_UNSAFE;
 		} else {
 			int handle = xamarin_create_ns_exception (ns_exception, &exception_gchandle);
 			if (exception_gchandle != 0) {
diff --git a/runtime/trampolines.m b/runtime/trampolines.m
index bdab794b5c5c..cc3ffc879570 100644
--- a/runtime/trampolines.m
+++ b/runtime/trampolines.m
@@ -613,7 +613,7 @@
 xamarin_release_trampoline (id self, SEL sel)
 {
 	// COOP: does not access managed memory: any mode, but it assumes safe mode upon entry (it takes locks, and doesn't switch to safe mode).
-	MONO_ASSERT_GC_SAFE;
+	MONO_ASSERT_GC_SAFE_OR_DETACHED;
 	
 	int ref_count;
 	bool detach = false;
@@ -676,7 +676,7 @@
 xamarin_retain_trampoline (id self, SEL sel)
 {
 	// COOP: safe mode upon entry, switches to unsafe when acccessing managed memory.
-	MONO_ASSERT_GC_SAFE;
+	MONO_ASSERT_GC_SAFE_OR_DETACHED;
 
 	pthread_mutex_lock (&refcount_mutex);
 
diff --git a/tools/common/StaticRegistrar.cs b/tools/common/StaticRegistrar.cs
index c8aa9666e518..9591b211396a 100644
--- a/tools/common/StaticRegistrar.cs
+++ b/tools/common/StaticRegistrar.cs
@@ -3862,7 +3862,7 @@ void Specialize (AutoIndentStringBuilder sb, ObjCMethod method, List<Exception>
 			if (App.Embeddinator)
 				body.WriteLine ("xamarin_embeddinator_initialize ();");
 
-			body.WriteLine ("MONO_ASSERT_GC_SAFE;");
+			body.WriteLine ("MONO_ASSERT_GC_SAFE_OR_DETACHED;");
 			body.WriteLine ("MONO_THREAD_ATTACH;"); // COOP: this will switch to GC_UNSAFE
 			body.WriteLine ();