diff --git a/src/or/circuitmux.c b/src/or/circuitmux.c
index 29c3be17d5..71bc4efc4a 100644
--- a/src/or/circuitmux.c
+++ b/src/or/circuitmux.c
@@ -896,8 +896,8 @@ circuitmux_num_cells_for_circuit(circuitmux_t *cmux, circuit_t *circ)
  * Query total number of available cells on a circuitmux
  */
 
-unsigned int
-circuitmux_num_cells(circuitmux_t *cmux)
+MOCK_IMPL(unsigned int,
+circuitmux_num_cells, (circuitmux_t *cmux))
 {
   tor_assert(cmux);
 
diff --git a/src/or/circuitmux.h b/src/or/circuitmux.h
index 5833ee5eee..9a5ea7d675 100644
--- a/src/or/circuitmux.h
+++ b/src/or/circuitmux.h
@@ -120,7 +120,7 @@ int circuitmux_is_circuit_attached(circuitmux_t *cmux, circuit_t *circ);
 int circuitmux_is_circuit_active(circuitmux_t *cmux, circuit_t *circ);
 unsigned int circuitmux_num_cells_for_circuit(circuitmux_t *cmux,
                                               circuit_t *circ);
-unsigned int circuitmux_num_cells(circuitmux_t *cmux);
+MOCK_DECL(unsigned int, circuitmux_num_cells, (circuitmux_t *cmux));
 unsigned int circuitmux_num_circuits(circuitmux_t *cmux);
 unsigned int circuitmux_num_active_circuits(circuitmux_t *cmux);