diff --git a/src/applications/differential/storage/DifferentialInlineComment.php b/src/applications/differential/storage/DifferentialInlineComment.php
index 79336ad9c3..3b064adb45 100644
--- a/src/applications/differential/storage/DifferentialInlineComment.php
+++ b/src/applications/differential/storage/DifferentialInlineComment.php
@@ -126,7 +126,8 @@ final class DifferentialInlineComment
 
 
   public function getMarkupFieldKey($field) {
-    return 'DI:'.$this->getID();
+    // We can't use ID because synthetic comments don't have it.
+    return 'DI:'.PhabricatorHash::digest($this->getContent());
   }
 
   public function newMarkupEngine($field) {
diff --git a/src/infrastructure/markup/PhabricatorMarkupInterface.php b/src/infrastructure/markup/PhabricatorMarkupInterface.php
index a3c96449c2..06e191aae9 100644
--- a/src/infrastructure/markup/PhabricatorMarkupInterface.php
+++ b/src/infrastructure/markup/PhabricatorMarkupInterface.php
@@ -47,7 +47,7 @@ interface PhabricatorMarkupInterface {
    *   "{$phid}:{$field}"
    *
    * @param string Field name.
-   * @return string Cache key.
+   * @return string Cache key up to 125 characters.
    *
    * @task markup
    */