Index: src/GenPoly/Box.cc
===================================================================
--- src/GenPoly/Box.cc	(revision 5c52b06f5452e4062363f7830a32272f9136cfbc)
+++ src/GenPoly/Box.cc	(revision 9407ed8c7feb76668f5441a6967282abe079e8e4)
@@ -557,6 +557,6 @@
 			assert( ! arg->get_results().empty() );
 			if ( isPolyType( param, exprTyVars ) ) {
-				if ( dynamic_cast< TypeInstType *>( arg->get_results().front() ) ) {
-					// if the argument's type is a type parameter, we don't need to box again!
+				if ( isPolyType( arg->get_results().front() ) ) {
+					// if the argument's type is polymorphic, we don't need to box again!
 					return;
 				} else if ( arg->get_results().front()->get_isLvalue() ) {