diff --git a/RELNOTES b/RELNOTES
index e6b1a5f2..d5179f20 100644
--- a/RELNOTES
+++ b/RELNOTES
@@ -103,6 +103,10 @@ by Eric Young (eay@cryptsoft.com).
   he provided.
   [ISC-Bugs #38396]
 
+- Fix a small memory leak in the DHCPv6 version of the client code.
+  This is unlikely to cause significant issues in actual use.
+  [ISC-Bugs #40990]
+
 			Changes since 4.3.3b1
 - None
 
diff --git a/client/dhc6.c b/client/dhc6.c
index 41c27f46..37ce7394 100644
--- a/client/dhc6.c
+++ b/client/dhc6.c
@@ -418,6 +418,12 @@ valid_reply(struct packet *packet, struct client_state *client)
 		rval = ISC_FALSE;
 	}
 
+	/* clean up pointers to the strings */
+	if (sid.data != NULL)
+		data_string_forget(&sid, MDL);
+	if (cid.data != NULL)
+		data_string_forget(&cid, MDL);
+
 	return rval;
 }