diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 6df0a8d4997..b08d897b35c 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1570,10 +1570,10 @@ namespace ts {
         }
     }
 
-    export function formatStringFromArgs(text: string, args: { [index: number]: string; }, baseIndex?: number): string {
+    export function formatStringFromArgs(text: string, args: ArrayLike<string>, baseIndex?: number): string {
         baseIndex = baseIndex || 0;
 
-        return text.replace(/{(\d+)}/g, (_match, index?) => args[+index + baseIndex]);
+        return text.replace(/{(\d+)}/g, (_match, index?: string) => Debug.assertDefined(args[+index + baseIndex]));
     }
 
     export let localizedDiagnosticMessages: MapLike<string>;