diff --git a/src/healthkit.cs b/src/healthkit.cs
index b4bd3f50c114..2f8088525736 100644
--- a/src/healthkit.cs
+++ b/src/healthkit.cs
@@ -1914,7 +1914,7 @@ interface HKSample {
 		bool HasUndeterminedDuration { get; }
 	}
 
-	delegate void HKSampleQueryResultsHandler (HKSampleQuery query, HKSample [] results, NSError error);
+	delegate void HKSampleQueryResultsHandler (HKSampleQuery query, [NullAllowed] HKSample [] results, [NullAllowed] NSError error);
 
 	[iOS (8, 0)]
 	[Mac (13, 0)]