diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index d6fd4bbde605bd..c632d9f88b4dd8 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1956,11 +1956,11 @@ int cmd_index_pack(int argc,
 					warning(_("no threads support, ignoring %s"), arg);
 					nr_threads = 1;
 				}
-			} else if (starts_with(arg, "--pack_header=")) {
-				if (parse_pack_header_option(arg + 14,
+			} else if (skip_prefix(arg, "--pack_header=", &arg)) {
+				if (parse_pack_header_option(arg,
 							     input_buffer,
 							     &input_len) < 0)
-					die(_("bad %s"), arg);
+					die(_("bad --pack_header: %s"), arg);
 			} else if (!strcmp(arg, "-v")) {
 				verbose = 1;
 			} else if (!strcmp(arg, "--progress-title")) {
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 76c6a9031b782c..51a856d8233eb5 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -646,10 +646,10 @@ int cmd_unpack_objects(int argc,
 				fsck_set_msg_types(&fsck_options, arg);
 				continue;
 			}
-			if (starts_with(arg, "--pack_header=")) {
-				if (parse_pack_header_option(arg + 14,
+			if (skip_prefix(arg, "--pack_header=", &arg)) {
+				if (parse_pack_header_option(arg,
 							     buffer, &len) < 0)
-					die(_("bad %s"), arg);
+					die(_("bad --pack_header: %s"), arg);
 				continue;
 			}
 			if (skip_prefix(arg, "--max-input-size=", &arg)) {