diff --git a/src/tbf.cpp b/src/tbf.cpp
index 457fe24..2e6da02 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -887,6 +887,8 @@
 	}
 
 	old_ms = bts->ms_by_tlli(new_tlli);
+	/* Keep the old MS object for the update_ms() */
+	GprsMs::Guard guard(old_ms);
 	if (old_ms) {
 		/* Get them before calling set_ms() */
 		dl_tbf = old_ms->dl_tbf();
@@ -897,7 +899,7 @@
 
 		/* there might be an active and valid downlink TBF */
 		if (!ms()->dl_tbf() && dl_tbf)
-			/* Move it to the current MS */
+			/* Move it to the current MS (see the guard above) */
 			dl_tbf->set_ms(ms());
 	}