diff --git a/src/repository/api/ArcanistGitAPI.php b/src/repository/api/ArcanistGitAPI.php
--- a/src/repository/api/ArcanistGitAPI.php
+++ b/src/repository/api/ArcanistGitAPI.php
@@ -1202,4 +1202,9 @@
     $this->execxLocal('stash pop');
   }
 
+  protected function didReloadCommitRange() {
+    // After an amend, the symbolic head may resolve to a different commit.
+    $this->resolvedHeadCommit = null;
+  }
+
 }
diff --git a/src/workflow/ArcanistDiffWorkflow.php b/src/workflow/ArcanistDiffWorkflow.php
--- a/src/workflow/ArcanistDiffWorkflow.php
+++ b/src/workflow/ArcanistDiffWorkflow.php
@@ -1207,6 +1207,10 @@
       return false;
     }
 
+    if ($this->getArgument('head') !== null) {
+      return false;
+    }
+
     // Run this last: with --raw or --raw-command, we won't have a repository
     // API.
     if ($this->isHistoryImmutable()) {