diff --git a/reftable/block.c b/reftable/block.c
index 72eb73b380..ad9074dba6 100644
--- a/reftable/block.c
+++ b/reftable/block.c
@@ -291,9 +291,8 @@ static int restart_key_less(size_t idx, void *args)
 	/* the restart key is verbatim in the block, so this could avoid the
 	   alloc for decoding the key */
 	struct strbuf rkey = STRBUF_INIT;
-	struct strbuf last_key = STRBUF_INIT;
 	uint8_t unused_extra;
-	int n = reftable_decode_key(&rkey, &unused_extra, last_key, in);
+	int n = reftable_decode_key(&rkey, &unused_extra, in);
 	int result;
 	if (n < 0) {
 		a->error = 1;
@@ -326,35 +325,34 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 	if (it->next_off >= it->br->block_len)
 		return 1;
 
-	n = reftable_decode_key(&it->key, &extra, it->last_key, in);
+	n = reftable_decode_key(&it->last_key, &extra, in);
 	if (n < 0)
 		return -1;
-
-	if (!it->key.len)
+	if (!it->last_key.len)
 		return REFTABLE_FORMAT_ERROR;
 
 	string_view_consume(&in, n);
-	n = reftable_record_decode(rec, it->key, extra, in, it->br->hash_size);
+	n = reftable_record_decode(rec, it->last_key, extra, in, it->br->hash_size);
 	if (n < 0)
 		return -1;
 	string_view_consume(&in, n);
 
-	strbuf_swap(&it->last_key, &it->key);
 	it->next_off += start.len - in.len;
 	return 0;
 }
 
 int block_reader_first_key(struct block_reader *br, struct strbuf *key)
 {
-	struct strbuf empty = STRBUF_INIT;
-	int off = br->header_off + 4;
+	int off = br->header_off + 4, n;
 	struct string_view in = {
 		.buf = br->block.data + off,
 		.len = br->block_len - off,
 	};
-
 	uint8_t extra = 0;
-	int n = reftable_decode_key(key, &extra, empty, in);
+
+	strbuf_reset(key);
+
+	n = reftable_decode_key(key, &extra, in);
 	if (n < 0)
 		return n;
 	if (!key->len)
@@ -371,7 +369,6 @@ int block_iter_seek(struct block_iter *it, struct strbuf *want)
 void block_iter_close(struct block_iter *it)
 {
 	strbuf_release(&it->last_key);
-	strbuf_release(&it->key);
 }
 
 int block_reader_seek(struct block_reader *br, struct block_iter *it,
@@ -408,8 +405,8 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
 		if (err < 0)
 			goto done;
 
-		reftable_record_key(&rec, &it->key);
-		if (err > 0 || strbuf_cmp(&it->key, want) >= 0) {
+		reftable_record_key(&rec, &it->last_key);
+		if (err > 0 || strbuf_cmp(&it->last_key, want) >= 0) {
 			err = 0;
 			goto done;
 		}
diff --git a/reftable/block.h b/reftable/block.h
index 17481e6331..51699af233 100644
--- a/reftable/block.h
+++ b/reftable/block.h
@@ -84,12 +84,10 @@ struct block_iter {
 
 	/* key for last entry we read. */
 	struct strbuf last_key;
-	struct strbuf key;
 };
 
 #define BLOCK_ITER_INIT { \
 	.last_key = STRBUF_INIT, \
-	.key = STRBUF_INIT, \
 }
 
 /* initializes a block reader. */
diff --git a/reftable/record.c b/reftable/record.c
index 12a44f70e5..b9c6eee88a 100644
--- a/reftable/record.c
+++ b/reftable/record.c
@@ -159,20 +159,19 @@ int reftable_encode_key(int *restart, struct string_view dest,
 	return start.len - dest.len;
 }
 
-int reftable_decode_key(struct strbuf *key, uint8_t *extra,
-			struct strbuf last_key, struct string_view in)
+int reftable_decode_key(struct strbuf *last_key, uint8_t *extra,
+			struct string_view in)
 {
 	int start_len = in.len;
 	uint64_t prefix_len = 0;
 	uint64_t suffix_len = 0;
-	int n = get_var_int(&prefix_len, &in);
+	int n;
+
+	n = get_var_int(&prefix_len, &in);
 	if (n < 0)
 		return -1;
 	string_view_consume(&in, n);
 
-	if (prefix_len > last_key.len)
-		return -1;
-
 	n = get_var_int(&suffix_len, &in);
 	if (n <= 0)
 		return -1;
@@ -181,12 +180,12 @@ int reftable_decode_key(struct strbuf *key, uint8_t *extra,
 	*extra = (uint8_t)(suffix_len & 0x7);
 	suffix_len >>= 3;
 
-	if (in.len < suffix_len)
+	if (in.len < suffix_len ||
+	    prefix_len > last_key->len)
 		return -1;
 
-	strbuf_reset(key);
-	strbuf_add(key, last_key.buf, prefix_len);
-	strbuf_add(key, in.buf, suffix_len);
+	strbuf_setlen(last_key, prefix_len);
+	strbuf_add(last_key, in.buf, suffix_len);
 	string_view_consume(&in, suffix_len);
 
 	return start_len - in.len;
diff --git a/reftable/record.h b/reftable/record.h
index a05e2be179..91c9c6ebfd 100644
--- a/reftable/record.h
+++ b/reftable/record.h
@@ -81,9 +81,12 @@ int reftable_encode_key(int *is_restart, struct string_view dest,
 			struct strbuf prev_key, struct strbuf key,
 			uint8_t extra);
 
-/* Decode into `key` and `extra` from `in` */
-int reftable_decode_key(struct strbuf *key, uint8_t *extra,
-			struct strbuf last_key, struct string_view in);
+/*
+ * Decode into `last_key` and `extra` from `in`. `last_key` is expected to
+ * contain the decoded key of the preceding record, if any.
+ */
+int reftable_decode_key(struct strbuf *last_key, uint8_t *extra,
+			struct string_view in);
 
 /* reftable_index_record are used internally to speed up lookups. */
 struct reftable_index_record {
diff --git a/reftable/record_test.c b/reftable/record_test.c
index a86cff5526..89209894d8 100644
--- a/reftable/record_test.c
+++ b/reftable/record_test.c
@@ -295,7 +295,8 @@ static void test_key_roundtrip(void)
 	EXPECT(!restart);
 	EXPECT(n > 0);
 
-	m = reftable_decode_key(&roundtrip, &rt_extra, last_key, dest);
+	strbuf_addstr(&roundtrip, "refs/heads/master");
+	m = reftable_decode_key(&roundtrip, &rt_extra, dest);
 	EXPECT(n == m);
 	EXPECT(0 == strbuf_cmp(&key, &roundtrip));
 	EXPECT(rt_extra == extra);