VidCon, a three-day conference on online video, took place this weekend in Anaheim, California and attracted an estimated 75,000 attendees.

At a panel on LGBTQ activism and awareness, the moderator deadnamed transgender YouTuber Miles McKenna.

Deadnaming is the practice of referring to a transgender person by the name and pronouns they used before transitioning, and it's considered to be extremely derogatory.

The moderator — a producer for YouTube comedy duo Rhett & Link — has since apologized for making the "horrible mistake" of using McKenna's pre-transition name. VidCon itself also apologized.

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A transgender YouTuber is speaking out after his pre-transition name and pronouns were used this weekend at a panel spotlighting LGBTQ voices online.

Miles McKenna, a YouTuber with more than 1.1 million followers, spoke this weekend at VidCon, a conference about online video that attracted an estimated 75,000 teens excited to catch a glimpse of their favorite content creators. McKenna appeared Saturday on a panel with other queer YouTubers to discuss how they use their platforms for LGBTQ activism and education.

But during the event, the moderator deadnamed McKenna. Deadnaming is the practice of referring to a transgender person by the name and pronouns they used before transitioning, and it's considered to be extremely derogatory.

McKenna spoke out on Twitter about the incident, asking VidCon how they let someone moderate a panel who "doesn't know how to address trans people."

Read more: YouTube has lately struggled to protect its vulnerable creators. Analysts say the platform may lack a business incentive to do anything about it.

McKenna has been publicly out as transgender since early 2017, when he posted a video on YouTube titled, "So I'm Trans*." He also posted a video in April 2018 documenting his year-long journey taking testosterone as he transitioned.

"My issue is less with someone's ignorance and more with how was that person allowed to head a panel that partly focuses on trans topics," McKenna wrote on Twitter. "Who allowed me to be on the same stage, as well as allow me to invite my young trans audience saying it's a safe space."

Research and public discourse has shown that deadnaming transgender people can be harmful and invalidating, not to mention it could out people against their will. Actress Laverne Cox, a prominent advocate for transgender people, wrote on Twitter in 2018 that misgendering and deadnaming a transgender person is equivalent to an act of "cultural and structural violence."

Stevie Wynne Levine, the moderator of the LGBTQ panel, apologized to McKenna in a note on Twitter. Levine is an openly queer woman who acts as the executive producer behind Good Mythical Morning, a YouTube series hosted by comedy duo Rhett & Link.

Levine said in her note that she made a "horrible mistake" by misgendering McKenna when telling a story about the YouTuber from 2014, before McKenna went through transition.

"I regret not specifically asking you if I could reference your deadname in relation to the story, but the truth is- I really, really didn't know," Levine wrote on Twitter. "Please understand that my mistake was in that context: not realizing that, even when permissibly discussing your pre-transition life, I should not have used your deadname. Hopefully you can see the innocence of my mistake, and how it differs from the intentional and malicious attacks that members of the LGBTQ+ community still must face every day."

The panel that McKenna appeared on was presented by the Trevor Project, a nonprofit focusing on suicide prevention among LGBTQ youth. This is the first year that the Trevor Project has partnered with VidCon, and McKenna says the organization reached out "immediately" to him to apologize.

The Trevor Project also issued a statement on Sunday, writing on Twitter that McKenna's deadnaming "does not reflect our values, mission, or the youth we serve."

VidCon has since released its own apology that called the deadnaming of McKenna a "mistake."

"VidCon prides itself on being a safe and trusted space for all of our Featured Creators. We do our best to research and stay vigilant in what is appropriate in each of the communities that are represented in this diverse online video space," VidCon said in a statement to Business Insider. "A mistake was made [Saturday] that left @TheMilesMcKenna hurt. We have reached out to him directly to discuss it. We are committed to trying to eliminate instances like this moving forward and are so sorry that this happened."

One of the founders of VidCon, Hank Green of the Vlogbrothers channel, also reached out to McKenna. Green wrote on Twitter that the harmful effect of deadnaming transgender people is "a thing that I literally did not know until today."

"I have lots of trans/non-binary friends who have changed names and/or pronouns in the time I've known them, and I never really knew what to do when referring to the times when I knew them before they transitioned," Green wrote on Twitter. "I'm sorry if this should have been painfully obvious but, I just didn't know and I was afraid to ask."