Index: src/SymTab/Validate.cc
===================================================================
--- src/SymTab/Validate.cc	(revision 48fa82401317bb7f5eb3e1e71cabcefc6026e478)
+++ src/SymTab/Validate.cc	(revision a94b8291fd5d45ed741b2c85713c868ddc3b2bbc)
@@ -535,5 +535,8 @@
 		// need to carry over the 'sized' status of each decl in the instance
 		for ( auto p : group_iterate( traitDecl->get_parameters(), traitInst->get_parameters() ) ) {
-			TypeExpr * expr = strict_dynamic_cast< TypeExpr * >( std::get<1>(p) );
+			TypeExpr * expr = dynamic_cast< TypeExpr * >( std::get<1>(p) );
+			if ( ! expr ) {
+				throw SemanticError( "Expression parameters for trait instances are currently unsupported: ", std::get<1>(p) );
+			}
 			if ( TypeInstType * inst = dynamic_cast< TypeInstType * >( expr->get_type() ) ) {
 				TypeDecl * formalDecl = std::get<0>(p);