diff --git a/src/uart.cpp b/src/uart.cpp
index 660c5ed..2f18412 100644
--- a/src/uart.cpp
+++ b/src/uart.cpp
@@ -177,7 +177,7 @@ namespace sta
 			// First digit in buffer is 4 MSBs in value, so shift from high to low
 			uint8_t hex = ((value >> ((digits - 1 - i) * 4)) & 0xF);
 			if (hex > 9)
-				buffer[i] = 'A' + (hex - 9);
+				buffer[i] = 'A' + (hex - 10);
 			else
 				buffer[i] = '0' + hex;
 		}