diff --git a/spi.h b/spi.h
index aa80c43..3050ccf 100644
--- a/spi.h
+++ b/spi.h
@@ -108,5 +108,6 @@
 /* Error codes */
 #define SPI_INVALID_OPCODE	-2
 #define SPI_INVALID_ADDRESS	-3
+#define SPI_INVALID_LENGTH	-4
 
 #endif		/* !__SPI_H__ */