diff --git a/midx-write.c b/midx-write.c
index 40abe3868c..51e1390164 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1021,6 +1021,20 @@ static bool midx_needs_update(struct multi_pack_index *midx, struct write_midx_c
 	struct strbuf buf = STRBUF_INIT;
 	bool needed = true;
 
+	/*
+	 * Ensure that we have a valid checksum before consulting the
+	 * exisiting MIDX in order to determine if we can avoid an
+	 * update.
+	 *
+	 * This is necessary because the given MIDX is loaded directly
+	 * from the object store (because we still compare our proposed
+	 * update to any on-disk MIDX regardless of whether or not we
+	 * have assigned "ctx.m") and is thus not guaranteed to have a
+	 * valid checksum.
+	 */
+	if (!midx_checksum_valid(midx))
+		goto out;
+
 	/*
 	 * Ignore incremental updates for now. The assumption is that any
 	 * incremental update would be either empty (in which case we will bail
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index b6622849db..faae98c7e7 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -563,7 +563,7 @@ test_expect_success 'git fsck suppresses MIDX output with --no-progress' '
 	! grep "Verifying object offsets" err
 '
 
-test_expect_failure 'corrupt MIDX is not reused' '
+test_expect_success 'corrupt MIDX is not reused' '
 	corrupt_midx_and_verify $MIDX_BYTE_OFFSET "\377" $objdir \
 		"incorrect object offset" &&
 	git multi-pack-index write 2>err &&