diff --git a/lib/SimpleSAML/Stats/Output.php b/lib/SimpleSAML/Stats/Output.php
index 0fa1dbbda0a3d82fd601a4f76f0430133387a5d5..7ce4562e39d1cda100c30c9153347bec63c55577 100644
--- a/lib/SimpleSAML/Stats/Output.php
+++ b/lib/SimpleSAML/Stats/Output.php
@@ -31,5 +31,5 @@ abstract class Output
      * @param array $data The event.
      * @return void
      */
-    abstract public function emit(array $data);
+    abstract public function emit(array $data) : void;
 }
diff --git a/modules/core/lib/Stats/Output/File.php b/modules/core/lib/Stats/Output/File.php
index 4ff860d053923fa2ba748e5746b556d372c22249..30563548cec0de87b6902f922961d4c5b081c2a3 100644
--- a/modules/core/lib/Stats/Output/File.php
+++ b/modules/core/lib/Stats/Output/File.php
@@ -85,7 +85,7 @@ class File extends \SimpleSAML\Stats\Output
      * @param array $data  The event.
      * @return void
      */
-    public function emit(array $data)
+    public function emit(array $data) : void
     {
         Assert::notNull($data['time']);
 
diff --git a/modules/core/lib/Stats/Output/Log.php b/modules/core/lib/Stats/Output/Log.php
index 3886ab09f653548e5611daf7344bc7c92913f980..4817c1c9ef5ed09f338ee2dc550e924014a42b68 100644
--- a/modules/core/lib/Stats/Output/Log.php
+++ b/modules/core/lib/Stats/Output/Log.php
@@ -43,7 +43,7 @@ class Log extends \SimpleSAML\Stats\Output
      * @param array $data  The event
      * @return void
      */
-    public function emit(array $data)
+    public function emit(array $data) : void
     {
         $str_data = json_encode($data);
         call_user_func($this->logger, 'EVENT ' . $str_data);