diff --git a/c_src/encoder.c b/c_src/encoder.c
index e864777..afc3c14 100644
--- a/c_src/encoder.c
+++ b/c_src/encoder.c
@@ -307,10 +307,7 @@ enc_string(Encoder* e, ERL_NIF_TERM val)
                     if(uval < 0) {
                         return 0;
                     }
-                    esc_extra += utf8_esc_len(uval);
-                    if(ulen < 0) {
-                        return 0;
-                    }
+                    esc_extra += utf8_esc_len(uval) - ulen;
                 }
                 i += ulen;
         }