diff --git a/src/Foundation/NSObject2.cs b/src/Foundation/NSObject2.cs
index c47ade9d6c03..1c64c34093c4 100644
--- a/src/Foundation/NSObject2.cs
+++ b/src/Foundation/NSObject2.cs
@@ -327,7 +327,7 @@ internal Flags FlagsInternal {
 #endif // !NET || !__MACOS__
 
 		[DllImport ("__Internal")]
-		static extern void xamarin_release_managed_ref (IntPtr handle, [MarshalAs (UnmanagedType.I1)] bool user_type);
+		static extern void xamarin_release_managed_ref (IntPtr handle, byte user_type);
 
 		static void RegisterToggleReference (NSObject obj, IntPtr handle, bool isCustomType)
 		{
@@ -404,8 +404,7 @@ private void InitializeObject (bool alloced)
 		}
 
 		[DllImport ("__Internal")]
-		[return: MarshalAs (UnmanagedType.I1)]
-		static extern bool xamarin_set_gchandle_with_flags_safe (IntPtr handle, IntPtr gchandle, XamarinGCHandleFlags flags);
+		static extern byte xamarin_set_gchandle_with_flags_safe (IntPtr handle, IntPtr gchandle, XamarinGCHandleFlags flags);
 
 		void CreateManagedRef (bool retain)
 		{
@@ -415,7 +414,7 @@ void CreateManagedRef (bool retain)
 				var flags = XamarinGCHandleFlags.HasManagedRef | XamarinGCHandleFlags.InitialSet | XamarinGCHandleFlags.WeakGCHandle;
 				var gchandle = GCHandle.Alloc (this, GCHandleType.WeakTrackResurrection);
 				var h = GCHandle.ToIntPtr (gchandle);
-				if (!xamarin_set_gchandle_with_flags_safe (handle, h, flags)) {
+				if (xamarin_set_gchandle_with_flags_safe (handle, h, flags) == 0) {
 					// A GCHandle already existed: this shouldn't happen, but let's handle it anyway.
 					Runtime.NSLog ($"Tried to create a managed reference from an object that already has a managed reference (type: {GetType ()})");
 					gchandle.Free ();
@@ -435,7 +434,7 @@ void ReleaseManagedRef ()
 				/* If we're a wrapper type, we need to unregister here, since we won't enter the release trampoline */
 				Runtime.NativeObjectHasDied (handle, this);
 			}
-			xamarin_release_managed_ref (handle, user_type);
+			xamarin_release_managed_ref (handle, user_type.AsByte ());
 			FreeData ();
 		}
 
diff --git a/src/Foundation/NSSearchPath.cs b/src/Foundation/NSSearchPath.cs
index efd3e5dd301f..ce89dabfc215 100644
--- a/src/Foundation/NSSearchPath.cs
+++ b/src/Foundation/NSSearchPath.cs
@@ -46,10 +46,10 @@ namespace Foundation {
 	public static class NSSearchPath {
 		public static string [] GetDirectories (NSSearchPathDirectory directory, NSSearchPathDomain domainMask, bool expandTilde = true)
 		{
-			return CFArray.StringArrayFromHandle (NSSearchPathForDirectoriesInDomains ((nuint) (ulong) directory, (nuint) (ulong) domainMask, expandTilde));
+			return CFArray.StringArrayFromHandle (NSSearchPathForDirectoriesInDomains ((nuint) (ulong) directory, (nuint) (ulong) domainMask, expandTilde.AsByte ()));
 		}
 
 		[DllImport (Constants.FoundationLibrary)]
-		static extern IntPtr NSSearchPathForDirectoriesInDomains (nuint directory, nuint domainMask, [MarshalAs (UnmanagedType.I1)] bool expandTilde);
+		static extern IntPtr NSSearchPathForDirectoriesInDomains (nuint directory, nuint domainMask, byte expandTilde);
 	}
 }
diff --git a/src/Foundation/NSThread.cs b/src/Foundation/NSThread.cs
index 7c9be3f46144..103e86b4217b 100644
--- a/src/Foundation/NSThread.cs
+++ b/src/Foundation/NSThread.cs
@@ -37,7 +37,7 @@ public static double Priority {
 		}
 
 		[DllImport ("__Internal")]
-		static extern IntPtr xamarin_init_nsthread (IntPtr handle, [MarshalAs (UnmanagedType.I1)] bool is_direct_binding, IntPtr target, IntPtr selector, IntPtr argument);
+		static extern IntPtr xamarin_init_nsthread (IntPtr handle, byte is_direct_binding, IntPtr target, IntPtr selector, IntPtr argument);
 
 		IntPtr InitNSThread (NSObject target, Selector selector, NSObject argument)
 		{
@@ -46,7 +46,7 @@ IntPtr InitNSThread (NSObject target, Selector selector, NSObject argument)
 			if (selector is null)
 				throw new ArgumentNullException ("selector");
 
-			return xamarin_init_nsthread (IsDirectBinding ? this.Handle : this.SuperHandle, IsDirectBinding, target.Handle, selector.Handle, argument is null ? IntPtr.Zero : argument.Handle);
+			return xamarin_init_nsthread (IsDirectBinding ? this.Handle : this.SuperHandle, IsDirectBinding.AsByte (), target.Handle, selector.Handle, argument is null ? IntPtr.Zero : argument.Handle);
 		}
 
 		[Export ("initWithTarget:selector:object:")]
diff --git a/tests/cecil-tests/BlittablePInvokes.KnownFailures.cs b/tests/cecil-tests/BlittablePInvokes.KnownFailures.cs
index 7a4dd5937c16..df47d18b697d 100644
--- a/tests/cecil-tests/BlittablePInvokes.KnownFailures.cs
+++ b/tests/cecil-tests/BlittablePInvokes.KnownFailures.cs
@@ -142,7 +142,6 @@ public partial class BlittablePInvokes {
 			"Security.SslStatus Security.SslContext::SSLRead(System.IntPtr,System.Byte*,System.IntPtr,System.IntPtr&)",
 			"Security.SslStatus Security.SslContext::SSLSetSessionOption(System.IntPtr,Security.SslSessionOption,System.Boolean)",
 			"Security.SslStatus Security.SslContext::SSLWrite(System.IntPtr,System.Byte*,System.IntPtr,System.IntPtr&)",
-			"System.Boolean Foundation.NSObject::xamarin_set_gchandle_with_flags_safe(System.IntPtr,System.IntPtr,Foundation.NSObject/XamarinGCHandleFlags)",
 			"System.Boolean Network.NWAdvertiseDescriptor::nw_advertise_descriptor_get_no_auto_rename(System.IntPtr)",
 			"System.Boolean Network.NWBrowserDescriptor::nw_browse_descriptor_get_include_txt_record(System.IntPtr)",
 			"System.Boolean Network.NWConnectionGroup::nw_connection_group_reinsert_extracted_connection(System.IntPtr,System.IntPtr)",
@@ -219,8 +218,6 @@ public partial class BlittablePInvokes {
 			"System.Int32 Security.SecCertificate::SecCertificateCopyEmailAddresses(System.IntPtr,System.IntPtr&)",
 			"System.Int32 Security.SslContext::SSLCopyALPNProtocols(System.IntPtr,System.IntPtr&)",
 			"System.Int32 Security.SslContext::SSLSetSessionTicketsEnabled(System.IntPtr,System.Boolean)",
-			"System.IntPtr Foundation.NSSearchPath::NSSearchPathForDirectoriesInDomains(System.UIntPtr,System.UIntPtr,System.Boolean)",
-			"System.IntPtr Foundation.NSThread::xamarin_init_nsthread(System.IntPtr,System.Boolean,System.IntPtr,System.IntPtr,System.IntPtr)",
 			"System.IntPtr ObjCRuntime.Selector::GetHandle(System.String)",
 			"System.IntPtr Security.SecAccessControl::SecAccessControlCreateWithFlags(System.IntPtr,System.IntPtr,System.IntPtr,System.IntPtr&)",
 			"System.IntPtr Security.SecCertificate::SecCertificateCopySerialNumberData(System.IntPtr,System.IntPtr&)",
@@ -232,7 +229,6 @@ public partial class BlittablePInvokes {
 			"System.IntPtr Security.SecKey::SecKeyCreateSignature(System.IntPtr,System.IntPtr,System.IntPtr,System.IntPtr&)",
 			"System.IntPtr Security.SecKey::SecKeyCreateWithData(System.IntPtr,System.IntPtr,System.IntPtr&)",
 			"System.IntPtr Security.SecPolicy::SecPolicyCreateSSL(System.Boolean,System.IntPtr)",
-			"System.Void Foundation.NSObject::xamarin_release_managed_ref(System.IntPtr,System.Boolean)",
 			"System.Void Network.NWAdvertiseDescriptor::nw_advertise_descriptor_set_no_auto_rename(System.IntPtr,System.Boolean)",
 			"System.Void Network.NWBrowserDescriptor::nw_browse_descriptor_set_include_txt_record(System.IntPtr,System.Boolean)",
 			"System.Void Network.NWConnection::nw_connection_send(System.IntPtr,System.IntPtr,System.IntPtr,System.Boolean,ObjCRuntime.BlockLiteral*)",