Expression.Lamba variable '' of type 'System.String' referenced from scope '', but it is not defined

I am trying to build a system that loads function delegates to a dictionary and then they can be called from anywhere in the environment asking the dictionary for the delegate. My functions are of a format func.

My code is

        var methods = typeof(Keywords).GetMethods().Where(mt => mt.GetCustomAttributes(typeof(KDTAttribute), false).Count() > 0);
        foreach (var method in methods)
        {
            string key = ((KDTAttribute)method.GetCustomAttributes(typeof(KDTAttribute), false)[0]).Keyword;
            var combinedArgumentsExp = new Expression[] { Expression.Parameter(typeof(string),"param") };
            var mtCall = Expression.Call(Expression.Constant(me), method,combinedArgumentsExp);
            ParameterExpression targetExpr = Expression.Parameter(typeof(string), "param");
            Func<string, string> result = Expression.Lambda<Func<string, string>>(mtCall, targetExpr).Compile();
            retVal.Add(key, result);

        }

I get the exception on the Expression.Lambda line : variable 'param' of type 'System.String' referenced from scope '', but it is not defined.

P.S If there is a better way to load delegates to a dictionary at run time ill be happy for any suggestions.