diff --git a/postinstall_runner_action.h b/postinstall_runner_action.h
index fea0a39..4e3e6e5 100644
--- a/postinstall_runner_action.h
+++ b/postinstall_runner_action.h
@@ -49,7 +49,7 @@
   const char* powerwash_marker_file_;
 
   // Special ctor + friend declaration for testing purposes.
-  PostinstallRunnerAction(const char* powerwash_marker_file)
+  explicit PostinstallRunnerAction(const char* powerwash_marker_file)
       : powerwash_marker_created_(false),
         powerwash_marker_file_(powerwash_marker_file) {}