diff --git a/src/or/channel.c b/src/or/channel.c
index ec44cce427..edeee6937a 100644
--- a/src/or/channel.c
+++ b/src/or/channel.c
@@ -2197,8 +2197,8 @@ channel_listener_change_state(channel_listener_t *chan_l,
 
 #define MAX_CELLS_TO_GET_FROM_CIRCUITS_FOR_UNLIMITED 256
 
-ssize_t
-channel_flush_some_cells(channel_t *chan, ssize_t num_cells)
+MOCK_IMPL(ssize_t,
+channel_flush_some_cells, (channel_t *chan, ssize_t num_cells))
 {
   unsigned int unlimited = 0;
   ssize_t flushed = 0;
diff --git a/src/or/channel.h b/src/or/channel.h
index 07a66ebcda..dd0ca369cd 100644
--- a/src/or/channel.h
+++ b/src/or/channel.h
@@ -455,7 +455,8 @@ void channel_queue_var_cell(channel_t *chan, var_cell_t *var_cell);
 void channel_flush_cells(channel_t *chan);
 
 /* Request from lower layer for more cells if available */
-ssize_t channel_flush_some_cells(channel_t *chan, ssize_t num_cells);
+MOCK_DECL(ssize_t, channel_flush_some_cells,
+          (channel_t *chan, ssize_t num_cells));
 
 /* Query if data available on this channel */
 int channel_more_to_flush(channel_t *chan);