diff --git a/t/unit-tests/t-reftable-basics.c b/t/unit-tests/t-reftable-basics.c
index a814e819756..a329f552025 100644
--- a/t/unit-tests/t-reftable-basics.c
+++ b/t/unit-tests/t-reftable-basics.c
@@ -113,8 +113,8 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 		};
 
 		for (size_t i = 0; i < ARRAY_SIZE(cases); i++) {
-			reftable_buf_addstr(&a, cases[i].a);
-			reftable_buf_addstr(&b, cases[i].b);
+			check(!reftable_buf_addstr(&a, cases[i].a));
+			check(!reftable_buf_addstr(&b, cases[i].b));
 			check_int(common_prefix_size(&a, &b), ==, cases[i].want);
 			reftable_buf_reset(&a);
 			reftable_buf_reset(&b);
diff --git a/t/unit-tests/t-reftable-block.c b/t/unit-tests/t-reftable-block.c
index df1d45fe8e4..f9af907117b 100644
--- a/t/unit-tests/t-reftable-block.c
+++ b/t/unit-tests/t-reftable-block.c
@@ -167,7 +167,7 @@ static void t_log_block_read_write(void)
 	for (i = 0; i < N; i++) {
 		block_iter_reset(&it);
 		reftable_buf_reset(&want);
-		reftable_buf_addstr(&want, recs[i].u.log.refname);
+		check(!reftable_buf_addstr(&want, recs[i].u.log.refname));
 
 		ret = block_iter_seek_key(&it, &br, &want);
 		check_int(ret, ==, 0);
@@ -314,7 +314,7 @@ static void t_index_block_read_write(void)
 
 		reftable_buf_init(&recs[i].u.idx.last_key);
 		recs[i].type = BLOCK_TYPE_INDEX;
-		reftable_buf_addstr(&recs[i].u.idx.last_key, buf);
+		check(!reftable_buf_addstr(&recs[i].u.idx.last_key, buf));
 		recs[i].u.idx.offset = i;
 
 		ret = block_writer_add(&bw, &recs[i]);
diff --git a/t/unit-tests/t-reftable-readwrite.c b/t/unit-tests/t-reftable-readwrite.c
index 7c7c72bb162..d279b86df0a 100644
--- a/t/unit-tests/t-reftable-readwrite.c
+++ b/t/unit-tests/t-reftable-readwrite.c
@@ -23,7 +23,7 @@ static void t_buffer(void)
 	struct reftable_block out = { 0 };
 	int n;
 	uint8_t in[] = "hello";
-	reftable_buf_add(&buf, in, sizeof(in));
+	check(!reftable_buf_add(&buf, in, sizeof(in)));
 	block_source_from_buf(&source, &buf);
 	check_int(block_source_size(&source), ==, 6);
 	n = block_source_read_block(&source, &out, 0, sizeof(in));
@@ -443,8 +443,8 @@ static void t_table_read_write_seek(int index, int hash_id)
 		reftable_iterator_destroy(&it);
 	}
 
-	reftable_buf_addstr(&pastLast, names[N - 1]);
-	reftable_buf_addstr(&pastLast, "/");
+	check(!reftable_buf_addstr(&pastLast, names[N - 1]));
+	check(!reftable_buf_addstr(&pastLast, "/"));
 
 	err = reftable_reader_init_ref_iterator(reader, &it);
 	check(!err);
@@ -901,7 +901,7 @@ static void t_corrupt_table(void)
 	struct reftable_block_source source = { 0 };
 	struct reftable_reader *reader;
 	int err;
-	reftable_buf_add(&buf, zeros, sizeof(zeros));
+	check(!reftable_buf_add(&buf, zeros, sizeof(zeros)));
 
 	block_source_from_buf(&source, &buf);
 	err = reftable_reader_new(&reader, &source, "file.log");
diff --git a/t/unit-tests/t-reftable-record.c b/t/unit-tests/t-reftable-record.c
index f2dd01688f3..eb98bf2da91 100644
--- a/t/unit-tests/t-reftable-record.c
+++ b/t/unit-tests/t-reftable-record.c
@@ -335,14 +335,14 @@ static void t_key_roundtrip(void)
 	int n, m;
 	uint8_t rt_extra;
 
-	reftable_buf_addstr(&last_key, "refs/heads/master");
-	reftable_buf_addstr(&key, "refs/tags/bla");
+	check(!reftable_buf_addstr(&last_key, "refs/heads/master"));
+	check(!reftable_buf_addstr(&key, "refs/tags/bla"));
 	extra = 6;
 	n = reftable_encode_key(&restart, dest, last_key, key, extra);
 	check(!restart);
 	check_int(n, >, 0);
 
-	reftable_buf_addstr(&roundtrip, "refs/heads/master");
+	check(!reftable_buf_addstr(&roundtrip, "refs/heads/master"));
 	m = reftable_decode_key(&roundtrip, &rt_extra, dest);
 	check_int(n, ==, m);
 	check(!reftable_buf_cmp(&key, &roundtrip));
@@ -469,9 +469,9 @@ static void t_reftable_index_record_comparison(void)
 			.u.idx.last_key = REFTABLE_BUF_INIT,
 		},
 	};
-	reftable_buf_addstr(&in[0].u.idx.last_key, "refs/heads/master");
-	reftable_buf_addstr(&in[1].u.idx.last_key, "refs/heads/master");
-	reftable_buf_addstr(&in[2].u.idx.last_key, "refs/heads/branch");
+	check(!reftable_buf_addstr(&in[0].u.idx.last_key, "refs/heads/master"));
+	check(!reftable_buf_addstr(&in[1].u.idx.last_key, "refs/heads/master"));
+	check(!reftable_buf_addstr(&in[2].u.idx.last_key, "refs/heads/branch"));
 
 	check(!reftable_record_equal(&in[0], &in[1], GIT_SHA1_RAWSZ));
 	check(!reftable_record_cmp(&in[0], &in[1]));
@@ -510,7 +510,7 @@ static void t_reftable_index_record_roundtrip(void)
 	int n, m;
 	uint8_t extra;
 
-	reftable_buf_addstr(&in.u.idx.last_key, "refs/heads/master");
+	check(!reftable_buf_addstr(&in.u.idx.last_key, "refs/heads/master"));
 	reftable_record_key(&in, &key);
 	t_copy(&in);
 
diff --git a/t/unit-tests/t-reftable-stack.c b/t/unit-tests/t-reftable-stack.c
index f49856270d6..72f6747064f 100644
--- a/t/unit-tests/t-reftable-stack.c
+++ b/t/unit-tests/t-reftable-stack.c
@@ -172,17 +172,17 @@ static void t_reftable_stack_add_one(void)
 	check_int(st->readers_len, >, 0);
 
 #ifndef GIT_WINDOWS_NATIVE
-	reftable_buf_addstr(&scratch, dir);
-	reftable_buf_addstr(&scratch, "/tables.list");
+	check(!reftable_buf_addstr(&scratch, dir));
+	check(!reftable_buf_addstr(&scratch, "/tables.list"));
 	err = stat(scratch.buf, &stat_result);
 	check(!err);
 	check_int((stat_result.st_mode & 0777), ==, opts.default_permissions);
 
 	reftable_buf_reset(&scratch);
-	reftable_buf_addstr(&scratch, dir);
-	reftable_buf_addstr(&scratch, "/");
+	check(!reftable_buf_addstr(&scratch, dir));
+	check(!reftable_buf_addstr(&scratch, "/"));
 	/* do not try at home; not an external API for reftable. */
-	reftable_buf_addstr(&scratch, st->readers[0]->name);
+	check(!reftable_buf_addstr(&scratch, st->readers[0]->name));
 	err = stat(scratch.buf, &stat_result);
 	check(!err);
 	check_int((stat_result.st_mode & 0777), ==, opts.default_permissions);
@@ -432,10 +432,10 @@ static void t_reftable_stack_auto_compaction_fails_gracefully(void)
 	 * Adding a new table to the stack should not be impacted by this, even
 	 * though auto-compaction will now fail.
 	 */
-	reftable_buf_addstr(&table_path, dir);
-	reftable_buf_addstr(&table_path, "/");
-	reftable_buf_addstr(&table_path, st->readers[0]->name);
-	reftable_buf_addstr(&table_path, ".lock");
+	check(!reftable_buf_addstr(&table_path, dir));
+	check(!reftable_buf_addstr(&table_path, "/"));
+	check(!reftable_buf_addstr(&table_path, st->readers[0]->name));
+	check(!reftable_buf_addstr(&table_path, ".lock"));
 	write_file_buf(table_path.buf, "", 0);
 
 	ref.update_index = 2;
@@ -575,17 +575,17 @@ static void t_reftable_stack_add(void)
 	}
 
 #ifndef GIT_WINDOWS_NATIVE
-	reftable_buf_addstr(&path, dir);
-	reftable_buf_addstr(&path, "/tables.list");
+	check(!reftable_buf_addstr(&path, dir));
+	check(!reftable_buf_addstr(&path, "/tables.list"));
 	err = stat(path.buf, &stat_result);
 	check(!err);
 	check_int((stat_result.st_mode & 0777), ==, opts.default_permissions);
 
 	reftable_buf_reset(&path);
-	reftable_buf_addstr(&path, dir);
-	reftable_buf_addstr(&path, "/");
+	check(!reftable_buf_addstr(&path, dir));
+	check(!reftable_buf_addstr(&path, "/"));
 	/* do not try at home; not an external API for reftable. */
-	reftable_buf_addstr(&path, st->readers[0]->name);
+	check(!reftable_buf_addstr(&path, st->readers[0]->name));
 	err = stat(path.buf, &stat_result);
 	check(!err);
 	check_int((stat_result.st_mode & 0777), ==, opts.default_permissions);
@@ -1078,10 +1078,10 @@ static void t_reftable_stack_auto_compaction_with_locked_tables(void)
 	 * size, we expect that auto-compaction will want to compact all of the
 	 * tables. Locking any of the tables will keep it from doing so.
 	 */
-	reftable_buf_addstr(&buf, dir);
-	reftable_buf_addstr(&buf, "/");
-	reftable_buf_addstr(&buf, st->readers[2]->name);
-	reftable_buf_addstr(&buf, ".lock");
+	check(!reftable_buf_addstr(&buf, dir));
+	check(!reftable_buf_addstr(&buf, "/"));
+	check(!reftable_buf_addstr(&buf, st->readers[2]->name));
+	check(!reftable_buf_addstr(&buf, ".lock"));
 	write_file_buf(buf.buf, "", 0);
 
 	/*
@@ -1164,10 +1164,10 @@ static void t_reftable_stack_compaction_with_locked_tables(void)
 	check_int(st->merged->readers_len, ==, 3);
 
 	/* Lock one of the tables that we're about to compact. */
-	reftable_buf_addstr(&buf, dir);
-	reftable_buf_addstr(&buf, "/");
-	reftable_buf_addstr(&buf, st->readers[1]->name);
-	reftable_buf_addstr(&buf, ".lock");
+	check(!reftable_buf_addstr(&buf, dir));
+	check(!reftable_buf_addstr(&buf, "/"));
+	check(!reftable_buf_addstr(&buf, st->readers[1]->name));
+	check(!reftable_buf_addstr(&buf, ".lock"));
 	write_file_buf(buf.buf, "", 0);
 
 	/*
@@ -1324,13 +1324,13 @@ static void t_reftable_stack_reload_with_missing_table(void)
 	 * our old readers. This should trigger a partial reload of the stack,
 	 * where we try to reuse our old readers.
 	*/
-	reftable_buf_addstr(&content, st->readers[0]->name);
-	reftable_buf_addstr(&content, "\n");
-	reftable_buf_addstr(&content, st->readers[1]->name);
-	reftable_buf_addstr(&content, "\n");
-	reftable_buf_addstr(&content, "garbage\n");
-	reftable_buf_addstr(&table_path, st->list_file);
-	reftable_buf_addstr(&table_path, ".lock");
+	check(!reftable_buf_addstr(&content, st->readers[0]->name));
+	check(!reftable_buf_addstr(&content, "\n"));
+	check(!reftable_buf_addstr(&content, st->readers[1]->name));
+	check(!reftable_buf_addstr(&content, "\n"));
+	check(!reftable_buf_addstr(&content, "garbage\n"));
+	check(!reftable_buf_addstr(&table_path, st->list_file));
+	check(!reftable_buf_addstr(&table_path, ".lock"));
 	write_file_buf(table_path.buf, content.buf, content.len);
 	err = rename(table_path.buf, st->list_file);
 	check(!err);
