diff --git a/builtin/patch-id.c b/builtin/patch-id.c
index 3894d2b970..583099cacf 100644
--- a/builtin/patch-id.c
+++ b/builtin/patch-id.c
@@ -5,6 +5,7 @@
 #include "hash.h"
 #include "hex.h"
 #include "parse-options.h"
+#include "setup.h"
 
 static void flush_current_id(int patchlen, struct object_id *id, struct object_id *result)
 {
@@ -237,6 +238,18 @@ int cmd_patch_id(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, builtin_patch_id_options,
 			     patch_id_usage, 0);
 
+	/*
+	 * We rely on `the_hash_algo` to compute patch IDs. This is dubious as
+	 * it means that the hash algorithm now depends on the object hash of
+	 * the repository, even though git-patch-id(1) clearly defines that
+	 * patch IDs always use SHA1.
+	 *
+	 * NEEDSWORK: This hack should be removed in favor of converting
+	 * the code that computes patch IDs to always use SHA1.
+	 */
+	if (!the_hash_algo)
+		repo_set_hash_algo(the_repository, GIT_HASH_SHA1);
+
 	generate_id_list(opts ? opts > 1 : config.stable,
 			 opts ? opts == 3 : config.verbatim);
 	return 0;
diff --git a/t/t1517-outside-repo.sh b/t/t1517-outside-repo.sh
index 389974d9fb..278ef57b3a 100755
--- a/t/t1517-outside-repo.sh
+++ b/t/t1517-outside-repo.sh
@@ -21,7 +21,7 @@ test_expect_success 'set up a non-repo directory and test file' '
 	git diff >sample.patch
 '
 
-test_expect_failure 'compute a patch-id outside repository (uses SHA-1)' '
+test_expect_success 'compute a patch-id outside repository (uses SHA-1)' '
 	nongit env GIT_DEFAULT_HASH=sha1 \
 		git patch-id <sample.patch >patch-id.expect &&
 	nongit \
diff --git a/t/t4204-patch-id.sh b/t/t4204-patch-id.sh
index a7fa94ce0a..605faea0c7 100755
--- a/t/t4204-patch-id.sh
+++ b/t/t4204-patch-id.sh
@@ -310,4 +310,38 @@ test_expect_success 'patch-id handles diffs with one line of before/after' '
 	test_config patchid.stable true &&
 	calc_patch_id diffu1stable <diffu1
 '
+
+test_expect_failure 'patch-id computes same ID with different object hashes' '
+	test_when_finished "rm -rf repo-sha1 repo-sha256" &&
+
+	cat >diff <<-\EOF &&
+	diff --git a/bar b/bar
+	index bdaf90f..31051f6 100644
+	--- a/bar
+	+++ b/bar
+	@@ -2 +2,2 @@
+	 b
+	+c
+	EOF
+
+	git init --object-format=sha1 repo-sha1 &&
+	git -C repo-sha1 patch-id <diff >patch-id-sha1 &&
+	git init --object-format=sha256 repo-sha256 &&
+	git -C repo-sha256 patch-id <diff >patch-id-sha256 &&
+	test_cmp patch-id-sha1 patch-id-sha256
+'
+
+test_expect_success 'patch-id without repository' '
+	cat >diff <<-\EOF &&
+	diff --git a/bar b/bar
+	index bdaf90f..31051f6 100644
+	--- a/bar
+	+++ b/bar
+	@@ -2 +2,2 @@
+	 b
+	+c
+	EOF
+	nongit git patch-id <diff
+'
+
 test_done