diff --git a/libraries/AP_Common/AP_Common.cpp b/libraries/AP_Common/AP_Common.cpp
index 4c7c3af0ce..8acdc2a76a 100644
--- a/libraries/AP_Common/AP_Common.cpp
+++ b/libraries/AP_Common/AP_Common.cpp
@@ -93,9 +93,9 @@ size_t strncpy_noterm(char *dest, const char *src, size_t n)
  * return the numeric value of an ascii hex character
  * 
  * @param[in] a Hexadecimal character 
- * @return  Returns a binary value
+ * @return  Returns a binary value.  If 'a' is not a valid hex character 255 (AKA -1) is returned
  */
-int16_t char_to_hex(char a)
+uint8_t char_to_hex(char a)
 {
     if (a >= 'A' && a <= 'F') {
         return a - 'A' + 10;
@@ -104,5 +104,5 @@ int16_t char_to_hex(char a)
     } else if (a >= '0' && a <= '9') {
         return a - '0';
     }
-    return 0;
+    return 255;
 }
diff --git a/libraries/AP_Common/AP_Common.h b/libraries/AP_Common/AP_Common.h
index d51e888e53..177a24c0fc 100644
--- a/libraries/AP_Common/AP_Common.h
+++ b/libraries/AP_Common/AP_Common.h
@@ -169,7 +169,7 @@ bool hex_to_uint8(uint8_t a, uint8_t &res);  // return the uint8 value of an asc
 size_t strncpy_noterm(char *dest, const char *src, size_t n);
 
 // return the numeric value of an ascii hex character
-int16_t char_to_hex(char a);
+uint8_t char_to_hex(char a);
 
 /*
   Bit manipulation