diff --git a/RxCocoa/Common/_RXDelegateProxy.h b/RxCocoa/Common/_RXDelegateProxy.h
index 03b93bd8..22a5a402 100644
--- a/RxCocoa/Common/_RXDelegateProxy.h
+++ b/RxCocoa/Common/_RXDelegateProxy.h
@@ -14,7 +14,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property (nonatomic, weak, readonly) id _forwardToDelegate;
 
--(void)_setForwardToDelegate:(id)forwardToDelegate retainDelegate:(BOOL)retainDelegate;
+-(void)_setForwardToDelegate:(id __nullable)forwardToDelegate retainDelegate:(BOOL)retainDelegate;
 
 -(BOOL)hasWiredImplementationForSelector:(SEL)selector;
 
diff --git a/RxCocoa/Common/_RXDelegateProxy.m b/RxCocoa/Common/_RXDelegateProxy.m
index e5fe3335..2cbffa5a 100644
--- a/RxCocoa/Common/_RXDelegateProxy.m
+++ b/RxCocoa/Common/_RXDelegateProxy.m
@@ -96,7 +96,7 @@ static NSMutableDictionary *forwardableSelectorsPerClass = nil;
     return __forwardToDelegate;
 }
 
--(void)_setForwardToDelegate:(id)forwardToDelegate retainDelegate:(BOOL)retainDelegate {
+-(void)_setForwardToDelegate:(id __nullable)forwardToDelegate retainDelegate:(BOOL)retainDelegate {
     __forwardToDelegate = forwardToDelegate;
     if (retainDelegate) {
         self.strongForwardDelegate = forwardToDelegate;