AmbiguousMatchException in Expression.PropertyOrField

I am using reflection to create a lambda function. It works with most items I try it with, however on one of the properties it keeps throwing an Ambiguous Match Exception.

The code looks like this. The error happens when it hits Expression.PropertyOrField. The property I am using is of type decimal?. I think it might have to do with the fact that it is a nullable type, but I'm not sure.

public static LambdaExpression CreateExpression(Type type, string propertyName, ref Type returnType)
{
    var param = Expression.Parameter(type, "x");
    Expression body = param;
    foreach (var member in propertyName.Split('.'))
    {
        body = Expression.PropertyOrField(body, member);
    }
    returnType = body.Type;
    return Expression.Lambda(body, param);
}