@@ -418,9 +418,9 @@
 					(*p)[1] ++;
 					return (*p)[0];
 				}
 				else
-					memo[memokey] = tuple(evlay.lift(new BottomValue, ctx, pos), 0);
+					memo[memokey] = tuple(cast(Value)new BottomValue, 0);
 
 				Value r = nonMemoizedRun();
 
 				int touched = memo[memokey][1];
@@ -466,12 +466,13 @@
 					throw genex!RuntimeException(pos,
 						text("only ", defLay, " layer can call native function: ", name));
 				T typed_args;
 				foreach(i, Ti; T) {
-					typed_args[i] = cast(Ti) ctx.get(text(i), ValueLayer, pos);
+					Value vi = ctx.get(text(i), ValueLayer, pos);
+					typed_args[i] = cast(Ti) vi;
 					if( typed_args[i] is null )
 						throw genex!RuntimeException(pos,
-							sprintf!"type mismatch on the argument %d of native function: %s"(i+1,name));
+							sprintf!"type mismatch on the argument %d of native function %s. %s required but %s passed."(i+1,name,typeid(Ti),vi));
 				}
 				try {
 					return dg(typed_args);
 				} catch( RuntimeException e ) {
@@ -484,8 +485,19 @@
 }
 
 version(unittest)
 	import polemy.runtime;
+
+unittest
+{
+	auto e = new Evaluator;
+	enrollRuntimeLibrary(e);
+	assert_eq( e.evalString(`
+@@foo(x){x*2};
+@foo "+" (x,y){x};
+@foo(3+4)
+`), new IntValue(6) );
+}
 
 unittest
 {
 	auto e = new Evaluator;