diff --git a/interp.c b/interp.c
index 05fab5b..c35cec4 100644
--- a/interp.c
+++ b/interp.c
@@ -24,7 +24,7 @@
  */
 
 static value_t vector_ref(value_t v, fixnum_t idx);
-static char byte_string_ref(value_t v, fixnum_t idx);
+static uint8_t byte_string_ref(value_t v, fixnum_t idx);
 static value_t struct_ref(value_t v, fixnum_t idx);
 
 static void vector_set(value_t v, fixnum_t idx, value_t newval);
@@ -150,7 +150,7 @@ static value_t vector_ref(value_t v, fixnum_t idx)
   return vec->elements[idx];
 }
 
-static char byte_string_ref(value_t v, fixnum_t idx)
+static uint8_t byte_string_ref(value_t v, fixnum_t idx)
 {
   byte_string_t *str = get_byte_string(v);
   release_assert((idx >= 0) && (idx < str->size));