diff --git a/lib/IRGen/GenCast.cpp b/lib/IRGen/GenCast.cpp
index f5d350f075ca3..6a2fa61080b5e 100644
--- a/lib/IRGen/GenCast.cpp
+++ b/lib/IRGen/GenCast.cpp
@@ -803,8 +803,35 @@ void irgen::emitScalarCheckedCast(IRGenFunction &IGF,
   assert(sourceType.isObject());
   assert(targetType.isObject());
 
-  if (auto sourceOptObjectType = sourceType.getOptionalObjectType()) {
+  llvm::BasicBlock *nilCheckBB = nullptr;
+  llvm::BasicBlock *nilMergeBB = nullptr;
+
+  // Merge the nil check and return the merged result: either nil or the value.
+  auto returnNilCheckedResult = [&](IRBuilder &Builder,
+                                    Explosion &nonNilResult) {
+    if (nilCheckBB) {
+      auto notNilBB = Builder.GetInsertBlock();
+      Builder.CreateBr(nilMergeBB);
+
+      Builder.emitBlock(nilMergeBB);
+      // Insert result phi.
+      Explosion result;
+      while (!nonNilResult.empty()) {
+        auto val = nonNilResult.claimNext();
+        auto valTy = cast<llvm::PointerType>(val->getType());
+        auto nil = llvm::ConstantPointerNull::get(valTy);
+        auto phi = Builder.CreatePHI(valTy, 2);
+        phi->addIncoming(nil, nilCheckBB);
+        phi->addIncoming(val, notNilBB);
+        result.add(phi);
+      }
+      out = std::move(result);
+    } else {
+      out = std::move(nonNilResult);
+    }
+  };
 
+  if (auto sourceOptObjectType = sourceType.getOptionalObjectType()) {
     // Translate the value from an enum representation to a possibly-null
     // representation.  Note that we assume that this projection is safe
     // for the particular case of an optional class-reference or metatype
@@ -816,6 +843,22 @@ void irgen::emitScalarCheckedCast(IRGenFunction &IGF,
     assert(value.empty());
     value = std::move(optValue);
     sourceType = sourceOptObjectType;
+
+    // We need a null-check because the runtime function can't handle null in
+    // some of the cases.
+    if (targetType.isExistentialType()) {
+      auto &Builder = IGF.Builder;
+      auto val = value.getAll()[0];
+      auto isNotNil = Builder.CreateICmpNE(
+          val, llvm::ConstantPointerNull::get(
+                   cast<llvm::PointerType>(val->getType())));
+      auto *isNotNilContBB = llvm::BasicBlock::Create(IGF.IGM.getLLVMContext());
+      nilMergeBB = llvm::BasicBlock::Create(IGF.IGM.getLLVMContext());
+      nilCheckBB = Builder.GetInsertBlock();
+      Builder.CreateCondBr(isNotNil, isNotNilContBB, nilMergeBB);
+
+      Builder.emitBlock(isNotNilContBB);
+    }
   }
 
   // If the source value is a metatype, either do a metatype-to-metatype
@@ -880,11 +923,14 @@ void irgen::emitScalarCheckedCast(IRGenFunction &IGF,
   }
 
   if (targetType.isExistentialType()) {
+    Explosion outRes;
     emitScalarExistentialDowncast(IGF, instance, sourceType, targetType,
-                                  mode, /*not a metatype*/ None, out);
+                                  mode, /*not a metatype*/ None, outRes);
+    returnNilCheckedResult(IGF.Builder, outRes);
     return;
   }
 
+  Explosion outRes;
   llvm::Value *result = emitClassDowncast(IGF, instance, targetType, mode);
   out.add(result);
 }
diff --git a/test/IRGen/casts.sil b/test/IRGen/casts.sil
index 1d8b0de90a438..65a63f21f5870 100644
--- a/test/IRGen/casts.sil
+++ b/test/IRGen/casts.sil
@@ -271,6 +271,31 @@ nay:
   unreachable
 }
 
+// CHECK: define swiftcc {{.*}} @checked_downcast_optional_class_to_ex([[INT]])
+// CHECK: entry:
+// CHECK:   [[V1:%.*]] = inttoptr [[INT]] %0 to %T5casts1AC*
+// CHECK:   [[V2:%.*]] = icmp ne %T5casts1AC* [[V1]], null
+// CHECK:   br i1 [[V2]], label %[[LBL:.*]], label
+// CHECK:    [[LBL]]:
+// CHECK:   [[V4:%.*]] = bitcast %T5casts1AC* [[V1]] to %swift.type**
+// CHECK:    load %swift.type*, %swift.type** [[V4]]
+sil @checked_downcast_optional_class_to_ex : $@convention(thin) (@guaranteed Optional<A>) -> @owned Optional<CP> {
+bb0(%0 : $Optional<A>):
+  checked_cast_br %0 : $Optional<A> to $CP, bb1, bb2
+
+bb1(%3 : $CP):
+  %4 = enum $Optional<CP>, #Optional.some!enumelt.1, %3 : $CP
+  retain_value %0 : $Optional<A>
+  br bb3(%4 : $Optional<CP>)
+
+bb2:
+  %7 = enum $Optional<CP>, #Optional.none!enumelt
+  br bb3(%7 : $Optional<CP>)
+
+bb3(%9 : $Optional<CP>):
+  return %9 : $Optional<CP>
+}
+
 // CHECK-LABEL: define{{( protected)?}} swiftcc void @checked_metatype_to_object_casts
 sil @checked_metatype_to_object_casts : $@convention(thin) <T> (@thick Any.Type) -> () {
 entry(%e : $@thick Any.Type):
diff --git a/test/Interpreter/casts.swift b/test/Interpreter/casts.swift
new file mode 100644
index 0000000000000..c063c8e8ed81a
--- /dev/null
+++ b/test/Interpreter/casts.swift
@@ -0,0 +1,78 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import StdlibUnittest
+import Foundation
+
+protocol P : class { }
+protocol C : class { }
+
+class Foo : NSObject {}
+var Casts = TestSuite("Casts")
+
+
+@inline(never)
+func castit<ObjectType>(_ o: NSObject?, _ t: ObjectType.Type) -> ObjectType? {
+  return o as? ObjectType
+}
+
+@inline(never)
+func castitExistential<ObjectType>(_ o: C?, _ t: ObjectType.Type) -> ObjectType? {
+  return o as? ObjectType
+}
+
+Casts.test("cast optional<nsobject> to protocol") {
+  if let obj = castit(nil, P.self) {
+    print("fail")
+    expectUnreachable()
+  } else {
+    print("success")
+  }
+}
+
+Casts.test("cast optional<nsobject> to protocol meta") {
+  if let obj = castit(nil, P.Type.self) {
+    print("fail")
+    expectUnreachable()
+  } else {
+    print("success")
+  }
+}
+Casts.test("cast optional<protocol> to protocol") {
+  if let obj = castitExistential(nil, P.self) {
+    print("fail")
+    expectUnreachable()
+  } else {
+    print("success")
+  }
+}
+
+Casts.test("cast optional<protocol> to class") {
+  if let obj = castitExistential(nil, Foo.self) {
+    print("fail")
+    expectUnreachable()
+  } else {
+    print("success")
+  }
+}
+
+Casts.test("cast optional<protocol> to protocol meta") {
+  if let obj = castitExistential(nil, P.Type.self) {
+    expectUnreachable()
+    print("fail")
+  } else {
+    print("success")
+  }
+}
+
+Casts.test("cast optional<protocol> to class meta") {
+  if let obj = castitExistential(nil, Foo.Type.self) {
+    expectUnreachable()
+    print("fail")
+  } else {
+    print("success")
+  }
+}
+
+runAllTests()