diff --git a/index.html b/index.html
index 6736e0d..169d16b 100644
--- a/index.html
+++ b/index.html
@@ -102,7 +102,7 @@
       // listeners
       box.addEventListener("input", (e) => {
 	var currentValue = e.target.value.substring(0, box.getAttribute("maxlength"));
-        if (pattrn !== null && !pattrn.test(paramValue)) {
+        if (pattrn === null || !pattrn.test(paramValue)) {
 	    e.target.value = lastValue;
 	} else {
             lastValue = currentValue;