diff --git a/ci/request-reviews/verify-base-branch.sh b/ci/request-reviews/verify-base-branch.sh
index 92ea10bc6719b..b23dcc18a0554 100755
--- a/ci/request-reviews/verify-base-branch.sh
+++ b/ci/request-reviews/verify-base-branch.sh
@@ -50,22 +50,24 @@ for branch in "${developmentBranches[@]}"; do
     echo "Checking for extra commits from branch $branch" >&2
 
     # The first ancestor of the PR head that already exists in the other branch
-    mergeBase=$(git -C "$localRepo" merge-base "$headRef" "$branch")
+    alreadyMerged=$(git -C "$localRepo" merge-base "$headRef" "$branch")
 
     # The number of commits that are reachable from the PR head, not reachable from the PRs base branch
     # (up to here this would be the number of commits in the PR itself),
     # but that are _also_ in the development branch we're testing against.
     # So, in other words, the number of commits that the PR includes from other development branches
-    count=$(git -C "$localRepo" rev-list --count "$mergeBase" ^"$baseBranch")
+    count=$(git -C "$localRepo" rev-list --count "$alreadyMerged" ^"$baseBranch")
 
     if (( count != 0 )); then
+        mergeBase=$(git -C "$localRepo" merge-base "$headRef" "$baseBranch")
+
         echo -en "\e[31m"
         echo "This PR's base branch is set to $baseBranch, but $count already-merged commits are included from the $branch branch."
         echo "To remedy this, first make sure you know the target branch for your changes: https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#branch-conventions"
         echo "- If the changes should go to the $branch branch instead, change the base branch accordingly:"
         echo "  https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-base-branch-of-a-pull-request"
         echo "- If the changes should really go to the $baseBranch branch, rebase your PR on top of the merge base with the $branch branch:"
-        echo "  git rebase --onto $mergeBase && git push --force-with-lease"
+        echo "  git rebase --onto $mergeBase $alreadyMerged && git push --force-with-lease"
         echo -en "\e[0m"
         exit 1
     fi