From 29d6d0a90664702ded201a51e6710247d187d147 Mon Sep 17 00:00:00 2001
From: gazoodle <47351872+gazoodle@users.noreply.github.com>
Date: Fri, 17 Jun 2022 02:35:25 +0100
Subject: [PATCH] Fix modbus user-defined function handling (#3527)

---
 esphome/components/modbus/modbus.cpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/esphome/components/modbus/modbus.cpp b/esphome/components/modbus/modbus.cpp
index 19b5e8019e..0928b0f6f3 100644
--- a/esphome/components/modbus/modbus.cpp
+++ b/esphome/components/modbus/modbus.cpp
@@ -76,7 +76,12 @@ bool Modbus::parse_modbus_byte_(uint8_t byte) {
     // installed, but wait, there is the CRC, and if we get a hit there is a good
     // chance that this is a complete message ... admittedly there is a small chance is
     // isn't but that is quite small given the purpose of the CRC in the first place
-    data_len = at;
+
+    // Fewer than 2 bytes can't calc CRC
+    if (at < 2)
+      return true;
+
+    data_len = at - 2;
     data_offset = 1;
 
     uint16_t computed_crc = crc16(raw, data_offset + data_len);