Representatives from most of America’s high-profile technology companies met with Donald Trump at Trump Tower today. But there was a conspicuous absence: Twitter.

The irony, of course, is that Twitter is Trump’s preferred communication platform. But Politico’s Nancy Scola reports that this wasn’t an oversight. The Trump transition team deliberately snubbed Twitter in retaliation for a pre-election dispute over a custom emoji advertising deal. The Trump campaign, for its part, denies the Politico report, arguing that Twitter was simply too small to make the cut.

The Trump campaign’s feud with Twitter began when the campaign wanted to pay Twitter to add an emoji of a bag of money flying away anytime a Twitter user typed the hashtag #CrookedHillary. Twitter has offered this kind of “custom emoji” service to a number of deep-pocketed advertising customers. But according to a senior Trump campaign official, Twitter CEO Jack Dorsey balked at the request, and the Trump campaign retaliated by canceling their advertising deal with Twitter.

It’s a sign of the tricky waters ad-funded tech companies must navigate as they try to sell ads to political campaigns. Twitter was perfectly happy to take money from the Trump campaign to promote his candidacy. But adding a cutesy emoji to the #CrookedHillary hashtag was evidently a step too far for Dorsey.

Custom emojis are a standard feature of big Twitter ad deals

If you’re a small business wanting to advertise on Twitter, you can take advantage of features like “promoted tweets,” basically conventional ads formatted in tweet form. But if you’re willing to spend a lot of money, Twitter offers some services that are more distinctive.

One example is the custom hashtag emoji. For example, right now if you tweet something with the hashtag #Empire in it, Twitter will automatically add an emoji that looks like a cookie with a crown on it:

Since writing that tweet, I have learned that Empire is a television show that’s broadcast on Fox. Fox presumably paid Twitter a lot of money — Adweek reports Twitter charges a minimum of $1 million for the service — for that emoji to show up on the site any time someone types the hashtag #Empire.

A number of brands have jumped on this bandwagon. Last year, Coca-Cola got Twitter to add an emoji of two Coke bottles clinking every time someone tweeted #ShareACoke. Disney made a deal so that emojis would appear after #C3PO, #StormTrooper, and #BB8. During the Super Bowl, Pepsi got a custom hashtag for #PepsiHalftime; Budweiser got ones for #GiveADamn and #BudLightParty. And the Super Bowl itself got one for #SB50.

The Trump campaign thought this was all great fun, so they wanted to buy a custom hashtag of their own, according to Gary Coby, the Trump campaign’s digital advertising director. He says they signed a $5 million advertising deal with Twitter that, among other things, allowed the Trump campaign to create a custom hashtag emoji.

Team Trump chose #CrookedHillary as the hashtag that would be blessed with a custom emoji, Coby writes, and they sent over this icon of a hand holding a bag of money:

Coby says Twitter initially approved this icon, but then balked when the Trump campaign asked to switch to this icon of a stick figure running away with a bag of money:

Twitter said no to this idea, Coby says, and also said the Trump campaign could no longer use the first submission either. Among other things, Coby said, Twitter claimed it was worried about a lawsuit from the Clinton campaign.

The Trump campaign tried again, this time sending Twitter’s ad representatives an icon showing a moneybag with wings. That hashtag emoji would have looked like this:

This time, Coby said, Twitter didn’t offer any excuses. Dorsey got on the phone and told the Trump campaign that Twitter was suspending the use of hashtag emojis for political campaigns.

However, Coby argues, “the only other campaign large enough to have this type of deal would have been the Clinton campaign and my contacts inside TW informed me that they did not have one in place.” So in practice, he says, Twitter was simply shutting down the Trump campaign’s use of the feature.

Some kinds of advertising don’t work well in politics

Obviously, if Twitter had approved the #CrookedHillary hashtag, it would have prompted an outcry from Clinton’s supporters. In the past, custom hashtag emojis have been used for positive messages like #ShareACoke or #BudLightParty, not to suggest that a prominent public figure had committed a crime.

In a statement emailed to Vox, Twitter didn’t dispute Coby’s account, but it says it now has a policy against allowing the use of hashtag emojis in political campaigns.

“We have had specific discussions with several political organizations, including the Trump campaign, regarding branded emojis as part of broad advertising campaigns on Twitter,” the spokesperson said. “We believe that political advertising merits a level of disclosure and transparency that branded political emojis do not meet, and we ultimately decided not to permit this particular format for any political advertising.”

With conventional banner ads or, in Twitter’s case, promoted tweets, it’s generally clear to users that they’re looking at a paid advertisement. But things aren’t so clear with a custom hashtag emoji. It could seem to users like Twitter itself was endorsing the implicit claim of the #CrookedHillary hashtag.

But it’s awkward to establish this rule midway through a presidential campaign after one of the candidates has already signed a deal for a custom hashtag emoji. And so although Trump has long been an enthusiastic user of Twitter the app, he doesn’t seem to have a great relationship with Twitter the company.

Watch: Twitter’s Jack Dorsey on Trump’s tweeting and election win