Index: src/GenPoly/Box.cpp
===================================================================
--- src/GenPoly/Box.cpp	(revision f9ad69dc6e86c3c08876734b6909ce8978446504)
+++ src/GenPoly/Box.cpp	(revision d3b33d562b54bf3ba5b7da8d7e32ae363017d5c7)
@@ -515,4 +515,6 @@
 		ast::FunctionType const * adaptee,
 		TypeVarMap const & typeVars ) {
+	assertf( ast::FixedArgs == adaptee->isVarArgs,
+		"Cannot adapt a varadic function, should have been checked." );
 	ast::FunctionType * adapter = ast::deepCopy( adaptee );
 	if ( isDynRet( adapter, typeVars ) ) {