diff --git a/src/libsystemd-bus/bus-message.c b/src/libsystemd-bus/bus-message.c
index c7cc32ec15..773a670119 100644
--- a/src/libsystemd-bus/bus-message.c
+++ b/src/libsystemd-bus/bus-message.c
@@ -302,6 +302,7 @@ int bus_message_from_malloc(
         m->n_iovec = 1;
         m->iovec[0].iov_base = buffer;
         m->iovec[0].iov_len = length;
+        m->size = length;
 
         r = message_parse_fields(m);
         if (r < 0)