diff --git a/config.ru b/config.ru
index ac9fde5dd74851a99f99144d4a97035aa90cc958..f265ccd1eb4f2641edc24a12d103376c5bccddb6 100644
--- a/config.ru
+++ b/config.ru
@@ -170,16 +170,23 @@ end
 
 class UnknownTransactions
 	def self.from(customer_id, address, tx_hashes)
+		self.for(
+			customer_id,
+			fetch_rows_for(address, tx_hashes).map { |row| row["transaction_id"] }
+		)
+	end
+
+	def self.fetch_rows_for(address, tx_hashes)
 		values = tx_hashes.map do |tx_hash|
 			"('#{DB.escape_string(tx_hash)}/#{DB.escape_string(address)}')"
 		end
-		rows = DB.exec_params(<<-SQL)
+		return [] unless values
+		DB.exec_params(<<-SQL)
 			SELECT transaction_id FROM
 				(VALUES #{values.join(',')}) AS t(transaction_id)
 				LEFT JOIN transactions USING (transaction_id)
 			WHERE transactions.transaction_id IS NULL
 		SQL
-		self.for(customer_id, rows.map { |row| row["transaction_id"] })
 	end
 
 	def self.for(customer_id, transaction_ids)