diff --git a/js/converter.js b/js/converter.js
index ac59a25..a67f21d 100644
--- a/js/converter.js
+++ b/js/converter.js
@@ -1,6 +1,7 @@
 // Hex → RGB
 export function hexToRgb(hex) {
   const h = hex.replace('#', '');
+  if (!/^[0-9a-fA-F]{3}$/.test(h) && !/^[0-9a-fA-F]{6}$/.test(h)) return null;
   const n = parseInt(h.length === 3
     ? h.split('').map(c => c + c).join('')
     : h, 16);
@@ -16,10 +17,10 @@ export function rgbToHex({ r, g, b }) {
 export function rgbToHsl({ r, g, b }) {
   r /= 255; g /= 255; b /= 255;
   const max = Math.max(r, g, b), min = Math.min(r, g, b);
-  let h, s;
+  let h = 0, s = 0;
   const l = (max + min) / 2;
   if (max === min) {
-    h = s = 0;
+    // h and s remain 0 (achromatic)
   } else {
     const d = max - min;
     s = l > 0.5 ? d / (2 - max - min) : d / (max + min);