diff --git a/drivers/i2c/busses/i2c-ismt.c b/drivers/i2c/busses/i2c-ismt.c
index 9af2337eb17a00f358b6386226c74260a852e1b0..22ffcb73c185f592d8b4e6bdbb1ede45cbeb4951 100644
--- a/drivers/i2c/busses/i2c-ismt.c
+++ b/drivers/i2c/busses/i2c-ismt.c
@@ -341,8 +341,10 @@ static int ismt_process_desc(const struct ismt_desc *desc,
 			break;
 		case I2C_SMBUS_BLOCK_DATA:
 		case I2C_SMBUS_I2C_BLOCK_DATA:
+			if (desc->rxbytes != dma_buffer[0] + 1)
+				return -EMSGSIZE;
+
 			memcpy(data->block, dma_buffer, desc->rxbytes);
-			data->block[0] = desc->rxbytes - 1;
 			break;
 		}
 		return 0;