diff --git a/src/core/dbus-manager.c b/src/core/dbus-manager.c
index fa85a5e698..59a7f2bd35 100644
--- a/src/core/dbus-manager.c
+++ b/src/core/dbus-manager.c
@@ -772,6 +772,7 @@ static int method_generic_unit_operation(
 
         assert(message);
         assert(m);
+        assert(handler);
 
         /* Read the first argument from the command and pass the operation to the specified per-unit
          * method. */