diff --git a/tangled_comment_post.py b/tangled_comment_post.py
index 7b11843..9f95104 100644
--- a/tangled_comment_post.py
+++ b/tangled_comment_post.py
@@ -41,7 +41,14 @@ def scrape_form(doc):
     uris = re.findall(r'name="subject-uri"[^>]*value="([^"]+)"', doc)
     idxs = re.findall(r'name="pull-round-idx"[^>]*value="([^"]+)"', doc)
     if not uris or not idxs:
-        sys.exit("could not find the comment form (is the cookie still valid?)")
+        # Two causes, and naming only one sends the reader in the wrong direction: a
+        # reviewer hitting this on a phantom pull will re-auth, succeed, and still fail.
+        sys.exit(
+            "could not find the comment form. Two causes, in likelihood order:\n"
+            "  1. THE PULL DOES NOT EXIST — a 404 page renders 200 and carries no form.\n"
+            "     Check the branch on the remote, not the pull number.\n"
+            "  2. the cookie has expired."
+        )
     return uris[-1], idxs[-1]
 
 
diff --git a/tangled_comments.py b/tangled_comments.py
index 6dc7619..f1b70b9 100644
--- a/tangled_comments.py
+++ b/tangled_comments.py
@@ -86,6 +86,20 @@ def main():
     if a.round is not None:
         url += f"/round/{a.round}"
     doc = fetch(url, cookie)
+
+    # A 404 page renders 200 and carries no pull record, so "0 comments" used to be
+    # indistinguishable from "no such pull". On 2026-08-22 that cost a false report that
+    # #428 was filed and unreviewed, and a reviewer's time chasing it. The post tool never
+    # had the defect because it MUST resolve a subject-uri to work at all — so require the
+    # same identifier here, and refuse rather than print a header for a phantom.
+    if not re.search(r'name="subject-uri"[^>]*value="(at://[^"]+)"', doc):
+        sys.exit(
+            f"pull #{a.pull} does not exist in {a.owner}/{a.repo} "
+            f"(no pull record on {url}).\n"
+            "  This is NOT an auth failure: the page rendered, it simply carries no pull.\n"
+            "  A pull that exists always yields a subject-uri."
+        )
+
     comments = parse_comments(doc)
 
     if a.json: