diff --git a/src/cluster.h b/src/cluster.h
index 08a11e9a..01e1ce21 100644
--- a/src/cluster.h
+++ b/src/cluster.h
@@ -88,7 +88,10 @@ typedef struct clusterNode {
     int numslots;   /* Number of slots handled by this node */
     int numslaves;  /* Number of slave nodes, if this is a master */
     struct clusterNode **slaves; /* pointers to slave nodes */
-    struct clusterNode *slaveof; /* pointer to the master node */
+    struct clusterNode *slaveof; /* pointer to the master node. Note that it
+                                    may be NULL even if the node is a slave
+                                    if we don't have the master node in our
+                                    tables. */
     mstime_t ping_sent;      /* Unix time we sent latest ping */
     mstime_t pong_received;  /* Unix time we received the pong */
     mstime_t fail_time;      /* Unix time when FAIL flag was set */