diff --git a/Sources/Foundation/NSString.swift b/Sources/Foundation/NSString.swift
index 11872e1593..dbe94c8b17 100644
--- a/Sources/Foundation/NSString.swift
+++ b/Sources/Foundation/NSString.swift
@@ -1644,3 +1644,24 @@ extension NSString : _StructTypeBridgeable {
         return _StructType._unconditionallyBridgeFromObjectiveC(self)
     }
 }
+
+extension NSString : CVarArg {
+    @inlinable // c-abi
+    public var _cVarArgEncoding: [Int] {
+        return _encodeBitsAsWords(unsafeBitCast(self, to: AnyObject.self))
+    }
+}
+
+#if !_runtime(_ObjC)
+extension String : CVarArg, _CVarArgObject {
+    @inlinable // c-abi
+    public var _cVarArgObject: CVarArg {
+        return NSString(string: self)
+    }
+
+    @inlinable // c-abi
+    public var _cVarArgEncoding: [Int] {
+        fatalError("_cVarArgEncoding must be called on NSString instead")
+    }
+}
+#endif
diff --git a/Tests/Foundation/Tests/TestNSString.swift b/Tests/Foundation/Tests/TestNSString.swift
index 90ecb00d05..57e56c69a2 100644
--- a/Tests/Foundation/Tests/TestNSString.swift
+++ b/Tests/Foundation/Tests/TestNSString.swift
@@ -813,6 +813,15 @@ class TestNSString: LoopbackServerTest {
         }
     }
 
+    func test_initializeWithFormat4() {
+        let argument: [CVarArg] = ["One", "Two", "Three"]
+        withVaList(argument) {
+            pointer in
+            let string = NSString(format: "Testing %@ %@ %@", arguments: pointer)
+            XCTAssertEqual(string, "Testing One Two Three")
+        }
+    }
+
     func test_appendingPathComponent() {
         do {
             let path: NSString = "/tmp"
@@ -1677,6 +1686,7 @@ class TestNSString: LoopbackServerTest {
             ("test_initializeWithFormat", test_initializeWithFormat),
             ("test_initializeWithFormat2", test_initializeWithFormat2),
             ("test_initializeWithFormat3", test_initializeWithFormat3),
+            ("test_initializeWithFormat4", test_initializeWithFormat4),
             ("test_appendingPathComponent", test_appendingPathComponent),
             ("test_deletingLastPathComponent", test_deletingLastPathComponent),
             ("test_getCString_simple", test_getCString_simple),