diff --git a/arduino_nano/lib/spi.c b/arduino_nano/lib/spi.c
index 81e8dbe..0640ddd 100644
--- a/arduino_nano/lib/spi.c
+++ b/arduino_nano/lib/spi.c
@@ -27,7 +27,7 @@
 #include <spi.h> // SPI configuration
 
 volatile uint8_t* spi_b = NULL; // the byte address to transmit/receive
-volatile size_t spi_i = NULL; // how many remaining bytes to transmit
+volatile size_t spi_i = 0; // how many remaining bytes to transmit
 
 /* receive SPI byte and transmit next on previous transmit completion when transfer is interrupt based (non-blocking) */
 ISR(SPI_STC_vect)