diff --git a/src/engine/thread.js b/src/engine/thread.js
index f7fa1dd7e..ceb643ea0 100644
--- a/src/engine/thread.js
+++ b/src/engine/thread.js
@@ -5,12 +5,13 @@
  */
 function Thread (firstBlock) {
     /**
-     * Top block of the thread
+     * ID of top block of the thread
+     * @type {!string}
      */
     this.topBlock = firstBlock;
     /**
-     * Next block that the thread will execute.
-     * @type {string}
+     * ID of next block that the thread will execute, or null if none.
+     * @type {?string}
      */
     this.nextBlock = firstBlock;
     /**