Cannot get last_post_time

I want to put a banner at the top of a thread if the last_post_time is older than three months.  Here's my code (just using a static message ID while testing):

<#assign cutoff = (.now?long - 90 * 86400000)?number_to_date />

<#assign msgQuery = "SELECT conversation.last_post_time FROM messages WHERE id='170' AND depth=0" />

<#assign msgInfo = rest("2.0", "/search?q=" + msgQuery?url) />

<#assign postDate = msgInfo.data.items.conversation.last_post_time?long />

<#list msgInfo.data.items as message>
    <#if postDate?number_to_date <= cutoff>
        <div id="post-closed">
            <p>This conversation is old. If you have a question please start a new post.</p>
        </div>
    </#if>
</#list>

No matter what I've tried, I can't seem to get the last_post_time value.  In the code above, it falls over at the <#assign postDate...> line.  It tells me:

The following has evaluated to null or missing

But testing the query in the API browser tells me the value is there!  I'm pretty sure I'm missing something rea-a-ally obvious but I've tried everything I can think of.

Can anyone help me out please?

Thanks in advance.

Paul