diff --git a/knockout/knockout.d.ts b/knockout/knockout.d.ts
index c20207199d..a906348a67 100644
--- a/knockout/knockout.d.ts
+++ b/knockout/knockout.d.ts
@@ -574,7 +574,7 @@ interface KnockoutComputedContext {
 declare module KnockoutComponentTypes {
 
     interface Config {
-        viewModel: ViewModelFunction | ViewModelSharedInstance | ViewModelFactoryFunction | AMDModule;
+        viewModel?: ViewModelFunction | ViewModelSharedInstance | ViewModelFactoryFunction | AMDModule;
         template: string | Node[]| DocumentFragment | TemplateElement | AMDModule;
     }