diff --git a/lib/chibi/json.c b/lib/chibi/json.c
index f47bfed8..d6d85342 100644
--- a/lib/chibi/json.c
+++ b/lib/chibi/json.c
@@ -415,7 +415,7 @@ sexp unparse_json_string(sexp ctx, sexp self, const sexp obj) {
           sprintf(cout, "\\t");
           break;
         default:
-          sprintf(cout, "%c", ch);
+          sprintf(cout, "%c", (int)ch);
           break;
       }
     } else if(ch <= 0xFFFF) {