diff --git a/src/core/or/circuitbuild.c b/src/core/or/circuitbuild.c
index 0527548f27..71a51e61a4 100644
--- a/src/core/or/circuitbuild.c
+++ b/src/core/or/circuitbuild.c
@@ -95,10 +95,10 @@ static const node_t *choose_good_middle_server(uint8_t purpose,
  * and then calls command_setup_channel() to give it the right
  * callbacks.
  */
-channel_t *
-channel_connect_for_circuit(const tor_addr_t *addr, uint16_t port,
-                            const char *id_digest,
-                            const struct ed25519_public_key_t *ed_id)
+MOCK_IMPL(channel_t *,
+channel_connect_for_circuit,(const tor_addr_t *addr, uint16_t port,
+                             const char *id_digest,
+                             const struct ed25519_public_key_t *ed_id))
 {
   channel_t *chan;
 
diff --git a/src/core/or/circuitbuild.h b/src/core/or/circuitbuild.h
index c66c0dc752..cf99990927 100644
--- a/src/core/or/circuitbuild.h
+++ b/src/core/or/circuitbuild.h
@@ -73,13 +73,13 @@ void circuit_upgrade_circuits_from_guard_wait(void);
 
 struct ed25519_public_key_t;
 
-channel_t *
-channel_connect_for_circuit(const tor_addr_t *addr,
-                            uint16_t port,
-                            const char *id_digest,
-                            const struct ed25519_public_key_t *ed_id);
-struct create_cell_t;
+MOCK_DECL(channel_t *,
+channel_connect_for_circuit,(const tor_addr_t *addr,
+                             uint16_t port,
+                             const char *id_digest,
+                             const struct ed25519_public_key_t *ed_id));
 
+struct create_cell_t;
 int
 circuit_deliver_create_cell(circuit_t *circ,
                             const struct create_cell_t *create_cell,