diff --git a/CHANGELOG.md b/CHANGELOG.md
index dc72536..f42b8e6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [[*next-version*]] - YYYY-MM-DD
+### Changed
+- **BC Breaking** - The return type of `ModuleInterface#run()` is now `void`.
+    This will break any subtypes that did not declare a return type,
+    because `void` is more specific than `mixed`.
 
 ## [0.2.0-alpha1] - 2020-04-10
 ### Changed
diff --git a/src/ModuleInterface.php b/src/ModuleInterface.php
index 1ad5c25..3e97c6e 100644
--- a/src/ModuleInterface.php
+++ b/src/ModuleInterface.php
@@ -48,5 +48,5 @@ public function setup() : ServiceProviderInterface;
      *
      * @throws ModuleExceptionInterface If the module failed to run.
      */
-    public function run(ContainerInterface $c);
+    public function run(ContainerInterface $c): void;
 }