diff --git a/src/blocked.c b/src/blocked.c
index ae2500aa..8acfb818 100644
--- a/src/blocked.c
+++ b/src/blocked.c
@@ -155,3 +155,27 @@ void replyToBlockedClientTimedOut(redisClient *c) {
     }
 }
 
+/* Mass-unblock clients because something changed in the instance that makes
+ * blocking no longer safe. For example clients blocked in list operations
+ * in an instance which turns from master to slave is unsafe, so this function
+ * is called when a master turns into a slave.
+ *
+ * The semantics is to send an -UNBLOCKED error to the client, disconnecting
+ * it at the same time. */
+void disconnectAllBlockedClients(void) {
+    listNode *ln;
+    listIter li;
+
+    listRewind(server.clients,&li);
+    while((ln = listNext(&li))) {
+        redisClient *c = listNodeValue(ln);
+
+        if (c->flags & REDIS_BLOCKED) {
+            addReplySds(c,sdsnew(
+                "-UNBLOCKED force unblock from blocking operation, "
+                "instance state changed (master -> slave?)\r\n"));
+            unblockClient(c);
+            c->flags |= REDIS_CLOSE_AFTER_REPLY;
+        }
+    }
+}
diff --git a/src/redis.h b/src/redis.h
index 34d8b0a4..53f3967d 100644
--- a/src/redis.h
+++ b/src/redis.h
@@ -1395,6 +1395,7 @@ void blockClient(redisClient *c, int btype);
 void unblockClient(redisClient *c);
 void replyToBlockedClientTimedOut(redisClient *c);
 int getTimeoutFromObjectOrReply(redisClient *c, robj *object, mstime_t *timeout, int unit);
+void disconnectAllBlockedClients(void);
 
 /* Git SHA1 */
 char *redisGitSHA1(void);
diff --git a/src/replication.c b/src/replication.c
index 697acbef..afea75b6 100644
--- a/src/replication.c
+++ b/src/replication.c
@@ -1444,6 +1444,7 @@ void replicationSetMaster(char *ip, int port) {
     server.masterhost = sdsnew(ip);
     server.masterport = port;
     if (server.master) freeClient(server.master);
+    disconnectAllBlockedClients(); /* Clients blocked in master, now slave. */
     disconnectSlaves(); /* Force our slaves to resync with us as well. */
     replicationDiscardCachedMaster(); /* Don't try a PSYNC. */
     freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */