diff --git a/src/libsystemd-network/dhcp6-internal.h b/src/libsystemd-network/dhcp6-internal.h
index 75bf879540..aefcdc2d36 100644
--- a/src/libsystemd-network/dhcp6-internal.h
+++ b/src/libsystemd-network/dhcp6-internal.h
@@ -173,6 +173,7 @@ const char *dhcp6_message_status_to_string(DHCP6Status s) _const_;
 DHCP6Status dhcp6_message_status_from_string(const char *s) _pure_;
 
 void dhcp6_client_set_test_mode(sd_dhcp6_client *client, bool test_mode);
+int dhcp6_client_set_transaction_id(sd_dhcp6_client *client, uint32_t transaction_id);
 
 #define log_dhcp6_client_errno(client, error, fmt, ...)         \
         log_interface_prefix_full_errno(                        \
diff --git a/src/libsystemd-network/fuzz-dhcp6-client-send.c b/src/libsystemd-network/fuzz-dhcp6-client-send.c
index 906c835b03..16a71e72d3 100644
--- a/src/libsystemd-network/fuzz-dhcp6-client-send.c
+++ b/src/libsystemd-network/fuzz-dhcp6-client-send.c
@@ -39,7 +39,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         assert_se(sd_dhcp6_client_set_information_request(client, false) == 0);
         dhcp6_client_set_test_mode(client, true);
         assert_se(sd_dhcp6_client_start(client) >= 0);
-        assert_se(sd_dhcp6_client_set_transaction_id(client, htobe32(0x00ffffff) & ((const DHCP6Message *) data)->transaction_id) == 0);
+        assert_se(dhcp6_client_set_transaction_id(client, ((const DHCP6Message *) data)->transaction_id) == 0);
 
         r = client_process_advertise_or_rapid_commit_reply(client, (DHCP6Message *) data, size, NULL, NULL);
         if (r < 0)
diff --git a/src/libsystemd-network/fuzz-dhcp6-client.c b/src/libsystemd-network/fuzz-dhcp6-client.c
index f62ab468df..10dbead21c 100644
--- a/src/libsystemd-network/fuzz-dhcp6-client.c
+++ b/src/libsystemd-network/fuzz-dhcp6-client.c
@@ -38,7 +38,7 @@ static void fuzz_client(const uint8_t *data, size_t size, bool is_information_re
         assert_se(sd_dhcp6_client_start(client) >= 0);
 
         if (size >= sizeof(DHCP6Message))
-                assert_se(sd_dhcp6_client_set_transaction_id(client, htobe32(0x00ffffff) & ((const DHCP6Message *) data)->transaction_id) == 0);
+                assert_se(dhcp6_client_set_transaction_id(client, ((const DHCP6Message *) data)->transaction_id) == 0);
 
         assert_se(write(test_dhcp_fd[1], data, size) == (ssize_t) size);
 
diff --git a/src/libsystemd-network/sd-dhcp6-client.c b/src/libsystemd-network/sd-dhcp6-client.c
index b59dce74a9..1d1e263306 100644
--- a/src/libsystemd-network/sd-dhcp6-client.c
+++ b/src/libsystemd-network/sd-dhcp6-client.c
@@ -592,10 +592,13 @@ int sd_dhcp6_client_set_address_request(sd_dhcp6_client *client, int request) {
         return 0;
 }
 
-int sd_dhcp6_client_set_transaction_id(sd_dhcp6_client *client, uint32_t transaction_id) {
-        assert_return(client, -EINVAL);
+int dhcp6_client_set_transaction_id(sd_dhcp6_client *client, uint32_t transaction_id) {
+        assert(client);
+        assert(client->test_mode);
 
-        client->transaction_id = transaction_id;
+        /* This is for tests or fuzzers. */
+
+        client->transaction_id = transaction_id & htobe32(0x00ffffff);
 
         return 0;
 }
diff --git a/src/systemd/sd-dhcp6-client.h b/src/systemd/sd-dhcp6-client.h
index d857af5acf..1bb21e0255 100644
--- a/src/systemd/sd-dhcp6-client.h
+++ b/src/systemd/sd-dhcp6-client.h
@@ -260,8 +260,6 @@ int sd_dhcp6_client_get_address_request(sd_dhcp6_client *client,
                                         int *request);
 int sd_dhcp6_client_set_address_request(sd_dhcp6_client *client,
                                         int request);
-int sd_dhcp6_client_set_transaction_id(sd_dhcp6_client *client,
-                                       uint32_t transaction_id);
 int sd_dhcp6_client_add_vendor_option(sd_dhcp6_client *client,
                                       sd_dhcp6_option *v);