diff --git a/ion/src/device/device.cpp b/ion/src/device/device.cpp
index 99b5e4e4d..f5e4b15a5 100644
--- a/ion/src/device/device.cpp
+++ b/ion/src/device/device.cpp
@@ -67,7 +67,7 @@ void Ion::reset() {
 
 static inline char hex(uint8_t d) {
   if (d > 9) {
-    return 'A'+d;
+    return 'A'+d-10;
   }
   return '0'+d;
 }