diff --git a/src/btstack_util.c b/src/btstack_util.c
index d4b464dd8..ba432b1f7 100644
--- a/src/btstack_util.c
+++ b/src/btstack_util.c
@@ -206,7 +206,7 @@ int16_t btstack_time16_delta(uint16_t time_a, uint16_t time_b){
     return (int16_t)(time_a - time_b);
 }
 
-char char_for_nibble(int nibble){
+char char_for_nibble(uint8_t nibble){
 
     static const char * char_to_nibble = "0123456789ABCDEF";
 
diff --git a/src/btstack_util.h b/src/btstack_util.h
index 26126d8cc..5ab94131a 100644
--- a/src/btstack_util.h
+++ b/src/btstack_util.h
@@ -206,7 +206,7 @@ bool btstack_is_null_bd_addr( const bd_addr_t addr );
  * @brief ASCII character for 4-bit nibble
  * @return character
  */
-char char_for_nibble(int nibble);
+char char_for_nibble(uint8_t nibble);
 
 /**
  * @brif 4-bit nibble from ASCII character