diff --git a/src/tarantool.c b/src/tarantool.c
index 4121018..ba46c65 100644
--- a/src/tarantool.c
+++ b/src/tarantool.c
@@ -1513,7 +1513,7 @@ PHP_METHOD(Tarantool, update) {
 		SSTR_LEN(obj->value) = before_len;
 		RETURN_FALSE;
 	}
-	php_tp_reencode_length(obj->value, sz);
+	php_tp_reencode_length(obj->value, before_len);
 	if (tarantool_stream_send(obj) == FAILURE)
 		RETURN_FALSE;
 
@@ -1543,7 +1543,7 @@ PHP_METHOD(Tarantool, upsert) {
 		SSTR_LEN(obj->value) = before_len;
 		RETURN_FALSE;
 	}
-	php_tp_reencode_length(obj->value, sz);
+	php_tp_reencode_length(obj->value, before_len);
 	if (tarantool_stream_send(obj) == FAILURE)
 		RETURN_FALSE;
 
diff --git a/src/tarantool_proto.c b/src/tarantool_proto.c
index b39e6cb..f8c69e5 100644
--- a/src/tarantool_proto.c
+++ b/src/tarantool_proto.c
@@ -268,9 +268,10 @@ void php_tp_encode_usplice(smart_string *str, uint32_t fieldno,
 	php_mp_pack_string(str, buffer, buffer_len);
 }
 
-void php_tp_reencode_length(smart_string *str, char *sz) {
-	ssize_t package_size = (SSTR_POS(str) - sz) - 5;
+void php_tp_reencode_length(smart_string *str, size_t orig_len) {
+	ssize_t package_size = (SSTR_LEN(str) - orig_len) - 5;
 	assert(package_size > 0);
+	char *sz = SSTR_BEG(str) + orig_len;
 	php_mp_pack_package_size_basic(sz, package_size);
 }
 
diff --git a/src/tarantool_proto.h b/src/tarantool_proto.h
index 8470923..658d0fb 100644
--- a/src/tarantool_proto.h
+++ b/src/tarantool_proto.h
@@ -150,7 +150,7 @@ void php_tp_encode_uother(smart_string *str, char type, uint32_t fieldno,
 void php_tp_encode_usplice(smart_string *str, uint32_t fieldno,
 			   uint32_t position, uint32_t offset,
 			   const char *buffer, size_t buffer_len);
-void php_tp_reencode_length(smart_string *str, char *sz);
+void php_tp_reencode_length(smart_string *str, size_t orig_len);
 
 int convert_iter_str(const char *i, size_t i_len);