diff --git a/src/libsystemd/sd-bus/bus-message.c b/src/libsystemd/sd-bus/bus-message.c
index 2c85130d198..ec3a39d051c 100644
--- a/src/libsystemd/sd-bus/bus-message.c
+++ b/src/libsystemd/sd-bus/bus-message.c
@@ -2063,6 +2063,7 @@ static int bus_message_close_variant(sd_bus_message *m, struct bus_container *c)
 
         assert(m);
         assert(c);
+        assert(c->signature);
 
         if (!BUS_MESSAGE_IS_GVARIANT(m))
                 return 0;