diff --git a/app/javascript/mastodon/features/ui/components/media_modal.js b/app/javascript/mastodon/features/ui/components/media_modal.js
index 23f588669..d869fffa6 100644
--- a/app/javascript/mastodon/features/ui/components/media_modal.js
+++ b/app/javascript/mastodon/features/ui/components/media_modal.js
@@ -35,7 +35,7 @@ export default class MediaModal extends ImmutablePureComponent {
   }
 
   handlePrevClick = () => {
-    this.setState({ index: (this.getIndex() - 1) % this.props.media.size });
+    this.setState({ index: (this.props.media.size + this.getIndex() - 1) % this.props.media.size });
   }
 
   handleKeyUp = (e) => {