--- a/src/luan/interp/ConcatExpr.java	Wed Nov 21 09:40:33 2012 +0000
+++ b/src/luan/interp/ConcatExpr.java	Thu Nov 22 04:37:10 2012 +0000
@@ -13,15 +13,13 @@
 	}
 
 	@Override Object eval(LuaState lua) throws LuaException {
-		Object v1 = op1.eval(lua);
-		Object v2 = op2.eval(lua);
-		check(v1);
-		check(v2);
-		return Lua.toString(v1) + Lua.toString(v2);
+		return toString(op1.eval(lua)) + toString(op2.eval(lua));
 	}
 
-	private static void check(Object v) throws LuaException {
-		if( !(v instanceof String || v instanceof LuaNumber) )
+	private static String toString(Object v) throws LuaException {
+		String s = Lua.asString(v);
+		if( s==null )
 			throw new LuaException( "attempt to concatenate a " + Lua.type(v) + " value" );
+		return s;
 	}
 }