diff --git a/src/digicode.js b/src/digicode.js
index cf31b02..de0208e 100644
--- a/src/digicode.js
+++ b/src/digicode.js
@@ -37,7 +37,9 @@ function composeDigit(digit) {
     clearTimeout(taskId);
     taskId = null;
   }
-  builder += digit.toString();
+  if (builder.length < CODE.length) {
+    builder += digit.toString();
+  }
   display.innerText = builder + '_'.repeat(CODE.length - builder.length);
   light.classList.remove(IS_SUCCESS, IS_ERROR);
   if (CODE.startsWith(builder)) {