diff --git a/lib/SILGen/SILGenPoly.cpp b/lib/SILGen/SILGenPoly.cpp
index cfefbb98000..2655878ab44 100644
--- a/lib/SILGen/SILGenPoly.cpp
+++ b/lib/SILGen/SILGenPoly.cpp
@@ -798,7 +798,7 @@ static CanSILFunctionType buildThunkType(SILGenFunction &gen,
   auto generics = gen.F.getLoweredFunctionType()->getGenericParams();
   if (generics) {
     for (auto archetype : generics->getAllArchetypes())
-      subs.push_back({ archetype, archetype});
+      subs.push_back({ archetype, archetype, { }});
   }
 
   // Add the function type as the parameter.