diff --git a/lib/client.js b/lib/client.js
index 18ed917f5..ad5a3b5b6 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -211,7 +211,9 @@ MatrixClient.prototype.setPowerLevel = function(roomId, userId, powerLevel,
         users: {}
     };
     if (event && event.getType() === "m.room.power_levels") {
-        content = event.getContent();
+        // take a copy of the content to ensure we don't corrupt
+        // existing client state with a failed power level change
+        content = utils.deepCopy(event.getContent());
     }
     content.users[userId] = powerLevel;
     var path = utils.encodeUri("/rooms/$roomId/state/m.room.power_levels", {