diff --git a/src/networking.c b/src/networking.c
index 3c754c376..85c640e34 100644
--- a/src/networking.c
+++ b/src/networking.c
@@ -2326,6 +2326,25 @@ NULL
                 return;
             }
 
+            if (options & CLIENT_TRACKING_OPTIN && options & CLIENT_TRACKING_OPTOUT)
+            {
+                addReplyError(c,
+                "You can't specify both OPTIN mode and OPTOUT mode");
+                zfree(prefix);
+                return;
+            }
+
+            if ((options & CLIENT_TRACKING_OPTIN && c->flags & CLIENT_TRACKING_OPTOUT) ||
+                (options & CLIENT_TRACKING_OPTOUT && c->flags & CLIENT_TRACKING_OPTIN))
+            {
+                addReplyError(c,
+                "You can't switch OPTIN/OPTOUT mode before disabling "
+                "tracking for this client, and then re-enabling it with "
+                "a different mode.");
+                zfree(prefix);
+                return;
+            }
+
             enableTracking(c,redir,options,prefix,numprefix);
         } else if (!strcasecmp(c->argv[2]->ptr,"off")) {
             disableTracking(c);