diff --git a/src/SpiFirmata.h b/src/SpiFirmata.h
index f405d04..9300d09 100644
--- a/src/SpiFirmata.h
+++ b/src/SpiFirmata.h
@@ -40,7 +40,7 @@
 struct spi_device_config {
   byte deviceIdChannel;
   byte csPinOptions;
-  byte csPin;
+  char csPin;
   boolean packedData;
   SPISettings spi_settings;
   boolean used;