diff --git a/src/darwin/Framework/CHIP/MTRAttributeValueWaiter.mm b/src/darwin/Framework/CHIP/MTRAttributeValueWaiter.mm
index 6be8a06e86c5d1..10d0b9f8e1393d 100644
--- a/src/darwin/Framework/CHIP/MTRAttributeValueWaiter.mm
+++ b/src/darwin/Framework/CHIP/MTRAttributeValueWaiter.mm
@@ -39,7 +39,7 @@ @interface MTRAttributeValueWaiter ()
 @property (nonatomic, retain) NSDictionary<MTRAttributePath *, MTRAwaitedAttributeState *> * valueExpectations;
 // Protected by the MTRDevice's lock.
 @property (nonatomic, readwrite, retain) dispatch_queue_t queue;
-@property (nonatomic, readwrite, retain, nullable) MTRStatusCompletion completion;
+@property (nonatomic, readwrite, copy, nullable) MTRStatusCompletion completion;
 @property (nonatomic, readonly, retain) MTRDevice * device;
 @end
 
diff --git a/src/darwin/Framework/CHIP/MTRDevice.mm b/src/darwin/Framework/CHIP/MTRDevice.mm
index 8cc9ab8550ff68..7c6414d2a1d9c0 100644
--- a/src/darwin/Framework/CHIP/MTRDevice.mm
+++ b/src/darwin/Framework/CHIP/MTRDevice.mm
@@ -742,7 +742,7 @@ - (MTRAttributeValueWaiter *)waitForAttributeValues:(NSDictionary<MTRAttributePa
     // Check whether the values coming in make sense.
     for (MTRAttributePath * path in values) {
         MTRVerifyArgumentOrDie(MTRDataValueDictionaryIsWellFormed(values[path]),
-            [NSString stringWithFormat:@"waitForAttributeValues handed invalid data-value %@ for path %@", path, values[path]]);
+            ([NSString stringWithFormat:@"waitForAttributeValues handed invalid data-value %@ for path %@", path, values[path]]));
     }
 
     // Check whether we have all these values already.