diff --git a/src/actions-util.ts b/src/actions-util.ts
index 4e4e772c3f..941e0ba01b 100644
--- a/src/actions-util.ts
+++ b/src/actions-util.ts
@@ -205,6 +205,24 @@ export const gitFetch = async function (branch: string, extraFlags: string[]) {
   }
 };
 
+/**
+ * Repack the git repository, using with the given flags. Errors are logged.
+ *
+ * This function uses the `checkout_path` to determine the repository path and
+ * works only when called from `analyze` or `upload-sarif`.
+ */
+export const gitRepack = async function (flags: string[]) {
+  try {
+    await runGitCommand(
+      getOptionalInput("checkout_path"),
+      ["repack", ...flags],
+      "Cannot repack the repository.",
+    );
+  } catch {
+    // Errors are already logged by runGitCommand()
+  }
+};
+
 /**
  * Compute the all merge bases between the given refs. Returns an empty array
  * if no merge base is found, or if there is an error.
diff --git a/src/analyze.ts b/src/analyze.ts
index fed0ae6c8e..a540527255 100644
--- a/src/analyze.ts
+++ b/src/analyze.ts
@@ -298,7 +298,7 @@ async function getPullRequestEditedDiffRanges(
 
   // To compute the merge bases between the base branch and the PR topic branch,
   // we need to fetch the commit graph from the branch heads to those merge
-  // babes. The following 4-step procedure does so while limiting the amount of
+  // babes. The following 6-step procedure does so while limiting the amount of
   // history fetched.
 
   // Step 1: Deepen from the PR merge commit to the base branch head and the PR
@@ -317,7 +317,12 @@ async function getPullRequestEditedDiffRanges(
   // Step 4: Fetch the base branch history, stopping when we reach commits that
   // are reachable from the PR topic branch head.
   await actionsUtil.gitFetch(baseRef, [`--shallow-exclude=${headRef}`]);
-  // Step 5: Deepen the history so that we have the merge bases between the base
+  // Step 5: Repack the history to remove the shallow grafts that were added by
+  // the previous fetches. This step works around a bug that causes subsequent
+  // deepening fetches to fail with "fatal: error in object: unshallow <SHA>".
+  // See https://stackoverflow.com/q/63878612
+  await actionsUtil.gitRepack(["-d"]);
+  // Step 6: Deepen the history so that we have the merge bases between the base
   // branch and the PR topic branch.
   await actionsUtil.deepenGitHistory();