diff --git a/src/components/Webmentions.astro b/src/components/Webmentions.astro
index 7e5523d..5fbc29d 100644
--- a/src/components/Webmentions.astro
+++ b/src/components/Webmentions.astro
@@ -570,7 +570,11 @@ const getEndpoint = endpoint ? getWebmentionEndpoint(endpoint, 'get') : '';
     fetch(form.action, {
       method: 'POST',
       body,
-      headers: { Accept: 'application/json' }
+      // webmentiond currently requires this Content-Type without a charset suffix.
+      headers: {
+        Accept: 'application/json',
+        'Content-Type': 'application/x-www-form-urlencoded'
+      }
     }).then(async (response) => {
       if (!response.ok) {
         const message = plainText(await response.text());