diff --git a/Foundation/NSDictionary.swift b/Foundation/NSDictionary.swift
index 18392f49d8..db0e8d6f66 100644
--- a/Foundation/NSDictionary.swift
+++ b/Foundation/NSDictionary.swift
@@ -586,7 +586,7 @@ open class NSMutableDictionary : NSDictionary {
         guard type(of: self) === NSDictionary.self || type(of: self) === NSMutableDictionary.self else {
             NSRequiresConcreteImplementation()
         }
-        _storage[(aKey as! NSObject)] = _SwiftValue.store(anObject)
+        _storage[_SwiftValue.store(aKey)] = _SwiftValue.store(anObject)
     }
     
     public convenience required init() {
diff --git a/TestFoundation/TestNSDictionary.swift b/TestFoundation/TestNSDictionary.swift
index b1435d4565..a5380de256 100644
--- a/TestFoundation/TestNSDictionary.swift
+++ b/TestFoundation/TestNSDictionary.swift
@@ -32,6 +32,7 @@ class TestNSDictionary : XCTestCase {
             ("test_mutableCopying", test_mutableCopying),
             ("test_writeToFile", test_writeToFile),
             ("test_initWithContentsOfFile", test_initWithContentsOfFile),
+            ("test_settingWithStringKey", test_settingWithStringKey),
         ]
     }
         
@@ -222,6 +223,12 @@ class TestNSDictionary : XCTestCase {
         }
     }
 
+    func test_settingWithStringKey() {
+        let dict = NSMutableDictionary()
+        // has crashed in the past
+        dict["stringKey"] = "value"
+    }
+
     private func createTestFile(_ path: String, _contents: Data) -> String? {
         let tempDir = NSTemporaryDirectory() + "TestFoundation_Playground_" + NSUUID().uuidString + "/"
         do {