diff --git a/src/btstack_util.c b/src/btstack_util.c
index 82bf075ae..d65b3844e 100644
--- a/src/btstack_util.c
+++ b/src/btstack_util.c
@@ -146,8 +146,8 @@ char char_for_nibble(int nibble){
 
 int nibble_for_char(char c){
     if (c >= '0' && c <= '9') return c - '0';
-    if (c >= 'a' && c <= 'f') return c - 'a';
-    if (c >= 'A' && c <= 'F') return c - 'F';
+    if (c >= 'a' && c <= 'f') return c - 'a' - 10;
+    if (c >= 'A' && c <= 'F') return c - 'A' - 10;
     return -1;
 }