diff --git a/ets2panda/checker/types/ets/etsFunctionType.cpp b/ets2panda/checker/types/ets/etsFunctionType.cpp
index 133c53eee87cf54b4f8942c4cc2f37457b5baf8b..2a963cb7376fdc0ce957b0575b2f83dad1e79a5b 100644
--- a/ets2panda/checker/types/ets/etsFunctionType.cpp
+++ b/ets2panda/checker/types/ets/etsFunctionType.cpp
@@ -84,7 +84,7 @@ static ETSObjectType *FunctionTypeToFunctionalInterfaceType(ETSChecker *checker,
         }
 
         auto substitution = Substitution {};
-        ES2PANDA_ASSERT(functionN != nullptr);
+        ES2PANDA_ASSERT(functionN != nullptr && nPosParams <= functionN->TypeArguments().size());
         for (size_t i = 0; i < nPosParams; i++) {
             substitution.emplace(functionN->TypeArguments()[i]->AsETSTypeParameter(),
                                  checker->MaybeBoxType(signature->Params()[i]->TsType()));