I use ExpandoObject for initialize Execute() method parameters. If take a look at SqlMapper.Execute() method, multiExec enabled for IEnumeable param, but it's unexpected behavior with ExpandoObject. Please, refine conditions like this:

if (multiExec != null && !(multiExec is string) &&
    // additional conditions:
    !(multiExec is IDynamicMetaObjectProvider) &&
    !(multiExec is IEnumerable<KeyValuePair<string, object>>))
{
    // execute as multiExec
}