diff --git a/builtin/log.c b/builtin/log.c
index e43ee12fb1..147850dc73 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1435,7 +1435,7 @@ static void prepare_bases(struct base_tree_info *bases,
 		struct object_id *patch_id;
 		if (*commit_base_at(&commit_base, commit))
 			continue;
-		if (commit_patch_id(commit, &diffopt, &oid, 0))
+		if (commit_patch_id(commit, &diffopt, &oid, 0, 1))
 			die(_("cannot get patch id"));
 		ALLOC_GROW(bases->patch_id, bases->nr_patch_id + 1, bases->alloc_patch_id);
 		patch_id = bases->patch_id + bases->nr_patch_id;
diff --git a/builtin/patch-id.c b/builtin/patch-id.c
index 970d0d30b4..bd28b80b2d 100644
--- a/builtin/patch-id.c
+++ b/builtin/patch-id.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "config.h"
+#include "diff.h"
 
 static void flush_current_id(int patchlen, struct object_id *id, struct object_id *result)
 {
@@ -54,22 +55,6 @@ static int scan_hunk_header(const char *p, int *p_before, int *p_after)
 	return 1;
 }
 
-static void flush_one_hunk(struct object_id *result, git_SHA_CTX *ctx)
-{
-	unsigned char hash[GIT_MAX_RAWSZ];
-	unsigned short carry = 0;
-	int i;
-
-	git_SHA1_Final(hash, ctx);
-	git_SHA1_Init(ctx);
-	/* 20-byte sum, with carry */
-	for (i = 0; i < GIT_SHA1_RAWSZ; ++i) {
-		carry += result->hash[i] + hash[i];
-		result->hash[i] = carry;
-		carry >>= 8;
-	}
-}
-
 static int get_one_patchid(struct object_id *next_oid, struct object_id *result,
 			   struct strbuf *line_buf, int stable)
 {
diff --git a/diff.c b/diff.c
index a654d46f6a..1ee04e321b 100644
--- a/diff.c
+++ b/diff.c
@@ -5990,6 +5990,22 @@ static int remove_space(char *line, int len)
 	return dst - line;
 }
 
+void flush_one_hunk(struct object_id *result, git_SHA_CTX *ctx)
+{
+	unsigned char hash[GIT_MAX_RAWSZ];
+	unsigned short carry = 0;
+	int i;
+
+	git_SHA1_Final(hash, ctx);
+	git_SHA1_Init(ctx);
+	/* 20-byte sum, with carry */
+	for (i = 0; i < GIT_SHA1_RAWSZ; ++i) {
+		carry += result->hash[i] + hash[i];
+		result->hash[i] = carry;
+		carry >>= 8;
+	}
+}
+
 static void patch_id_consume(void *priv, char *line, unsigned long len)
 {
 	struct patch_id_t *data = priv;
@@ -6014,8 +6030,8 @@ static void patch_id_add_mode(git_SHA_CTX *ctx, unsigned mode)
 	git_SHA1_Update(ctx, buf, len);
 }
 
-/* returns 0 upon success, and writes result into sha1 */
-static int diff_get_patch_id(struct diff_options *options, struct object_id *oid, int diff_header_only)
+/* returns 0 upon success, and writes result into oid */
+static int diff_get_patch_id(struct diff_options *options, struct object_id *oid, int diff_header_only, int stable)
 {
 	struct diff_queue_struct *q = &diff_queued_diff;
 	int i;
@@ -6025,6 +6041,7 @@ static int diff_get_patch_id(struct diff_options *options, struct object_id *oid
 	git_SHA1_Init(&ctx);
 	memset(&data, 0, sizeof(struct patch_id_t));
 	data.ctx = &ctx;
+	oidclr(oid);
 
 	for (i = 0; i < q->nr; i++) {
 		xpparam_t xpp;
@@ -6100,17 +6117,22 @@ static int diff_get_patch_id(struct diff_options *options, struct object_id *oid
 				  patch_id_consume, &data, &xpp, &xecfg))
 			return error("unable to generate patch-id diff for %s",
 				     p->one->path);
+
+		if (stable)
+			flush_one_hunk(oid, &ctx);
 	}
 
-	git_SHA1_Final(oid->hash, &ctx);
+	if (!stable)
+		git_SHA1_Final(oid->hash, &ctx);
+
 	return 0;
 }
 
-int diff_flush_patch_id(struct diff_options *options, struct object_id *oid, int diff_header_only)
+int diff_flush_patch_id(struct diff_options *options, struct object_id *oid, int diff_header_only, int stable)
 {
 	struct diff_queue_struct *q = &diff_queued_diff;
 	int i;
-	int result = diff_get_patch_id(options, oid, diff_header_only);
+	int result = diff_get_patch_id(options, oid, diff_header_only, stable);
 
 	for (i = 0; i < q->nr; i++)
 		diff_free_filepair(q->queue[i]);
diff --git a/diff.h b/diff.h
index d5e44baa96..b680b377b2 100644
--- a/diff.h
+++ b/diff.h
@@ -436,7 +436,8 @@ int run_diff_files(struct rev_info *revs, unsigned int option);
 int run_diff_index(struct rev_info *revs, int cached);
 
 int do_diff_cache(const struct object_id *, struct diff_options *);
-int diff_flush_patch_id(struct diff_options *, struct object_id *, int);
+int diff_flush_patch_id(struct diff_options *, struct object_id *, int, int);
+void flush_one_hunk(struct object_id *, git_SHA_CTX *);
 
 int diff_result_code(struct diff_options *, int);
 
diff --git a/patch-ids.c b/patch-ids.c
index c262e1be9c..f70d396654 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -11,7 +11,7 @@ static int patch_id_defined(struct commit *commit)
 }
 
 int commit_patch_id(struct commit *commit, struct diff_options *options,
-		    struct object_id *oid, int diff_header_only)
+		    struct object_id *oid, int diff_header_only, int stable)
 {
 	if (!patch_id_defined(commit))
 		return -1;
@@ -22,7 +22,7 @@ int commit_patch_id(struct commit *commit, struct diff_options *options,
 	else
 		diff_root_tree_oid(&commit->object.oid, "", options);
 	diffcore_std(options);
-	return diff_flush_patch_id(options, oid, diff_header_only);
+	return diff_flush_patch_id(options, oid, diff_header_only, stable);
 }
 
 /*
@@ -46,11 +46,11 @@ static int patch_id_neq(const void *cmpfn_data,
 	struct patch_id *b = (void *)entry_or_key;
 
 	if (is_null_oid(&a->patch_id) &&
-	    commit_patch_id(a->commit, opt, &a->patch_id, 0))
+	    commit_patch_id(a->commit, opt, &a->patch_id, 0, 0))
 		return error("Could not get patch ID for %s",
 			oid_to_hex(&a->commit->object.oid));
 	if (is_null_oid(&b->patch_id) &&
-	    commit_patch_id(b->commit, opt, &b->patch_id, 0))
+	    commit_patch_id(b->commit, opt, &b->patch_id, 0, 0))
 		return error("Could not get patch ID for %s",
 			oid_to_hex(&b->commit->object.oid));
 	return !oideq(&a->patch_id, &b->patch_id);
@@ -80,7 +80,7 @@ static int init_patch_id_entry(struct patch_id *patch,
 	struct object_id header_only_patch_id;
 
 	patch->commit = commit;
-	if (commit_patch_id(commit, &ids->diffopts, &header_only_patch_id, 1))
+	if (commit_patch_id(commit, &ids->diffopts, &header_only_patch_id, 1, 0))
 		return -1;
 
 	hashmap_entry_init(patch, sha1hash(header_only_patch_id.hash));
diff --git a/patch-ids.h b/patch-ids.h
index 82a12b66f8..03bb04e707 100644
--- a/patch-ids.h
+++ b/patch-ids.h
@@ -20,7 +20,7 @@ struct patch_ids {
 };
 
 int commit_patch_id(struct commit *commit, struct diff_options *options,
-		    struct object_id *oid, int);
+		    struct object_id *oid, int, int);
 int init_patch_ids(struct repository *, struct patch_ids *);
 int free_patch_ids(struct patch_ids *);
 struct patch_id *add_commit_patch_id(struct commit *, struct patch_ids *);
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index b6e2fdbc44..7a5833a4b2 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -36,8 +36,27 @@ test_expect_success setup '
 	git checkout master &&
 	git diff-tree -p C2 | git apply --index &&
 	test_tick &&
-	git commit -m "Master accepts moral equivalent of #2"
+	git commit -m "Master accepts moral equivalent of #2" &&
 
+	git checkout side &&
+	git checkout -b patchid &&
+	for i in 5 6 1 2 3 A 4 B C 7 8 9 10 D E F; do echo "$i"; done >file2 &&
+	for i in 1 2 3 A 4 B C 7 8 9 10 D E F 5 6; do echo "$i"; done >file3 &&
+	for i in 8 9 10; do echo "$i"; done >file &&
+	git add file file2 file3 &&
+	test_tick &&
+	git commit -m "patchid 1" &&
+	for i in 4 A B 7 8 9 10; do echo "$i"; done >file2 &&
+	for i in 8 9 10 5 6; do echo "$i"; done >file3 &&
+	git add file2 file3 &&
+	test_tick &&
+	git commit -m "patchid 2" &&
+	for i in 10 5 6; do echo "$i"; done >file &&
+	git add file &&
+	test_tick &&
+	git commit -m "patchid 3" &&
+
+	git checkout master
 '
 
 test_expect_success "format-patch --ignore-if-in-upstream" '
@@ -1559,7 +1578,7 @@ test_expect_success 'format-patch -o overrides format.outputDirectory' '
 '
 
 test_expect_success 'format-patch --base' '
-	git checkout side &&
+	git checkout patchid &&
 	git format-patch --stdout --base=HEAD~3 -1 | tail -n 7 >actual1 &&
 	git format-patch --stdout --base=HEAD~3 HEAD~.. | tail -n 7 >actual2 &&
 	echo >expected &&
@@ -1568,7 +1587,14 @@ test_expect_success 'format-patch --base' '
 	echo "prerequisite-patch-id: $(git show --patch HEAD~1 | git patch-id --stable | awk "{print \$1}")" >>expected &&
 	signature >> expected &&
 	test_cmp expected actual1 &&
-	test_cmp expected actual2
+	test_cmp expected actual2 &&
+	echo >fail &&
+	echo "base-commit: $(git rev-parse HEAD~3)" >>fail &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~2 | git patch-id --unstable | awk "{print \$1}")" >>fail &&
+	echo "prerequisite-patch-id: $(git show --patch HEAD~1 | git patch-id --unstable | awk "{print \$1}")" >>fail &&
+	signature >> fail &&
+	! test_cmp fail actual1 &&
+	! test_cmp fail actual2
 '
 
 test_expect_success 'format-patch --base errors out when base commit is in revision list' '