diff --git a/PluginMarket.class.php b/PluginMarket.class.php
index 83fc31a09edfcc0b476d592a457cd08b49c6ceee..e31737dea57724059b750eb559cbe4bf9f97df2e 100644
--- a/PluginMarket.class.php
+++ b/PluginMarket.class.php
@@ -83,7 +83,7 @@ class PluginMarket extends StudIPPlugin implements SystemPlugin, HomepagePlugin
 
             if ($follower['security_token']) {
                 $calculatedHash = hash_hmac("sha1", $payload, $follower['security_token']);
-                $header[] = "X_HUB_SIGNATURE: ".$calculatedHash;
+                $header[] = "X_HUB_SIGNATURE: sha1=".$calculatedHash;
             }
             $header[] = "Content-Type: application/json";