maybe this type check?

Author: cometkim

src/grapheme.js

@@ -45,15 +45,16 @@ const BMP_MAX = 0xFFFF;
  * @return {GraphemeSegmenter} iterator for grapheme cluster segments
  */
 export function* graphemeSegments(input) {
-  /** Total length of the input string. */
-  let len = input.length;
+  let cp = input.codePointAt(0);
 
   // do nothing on empty string
-  if (len === 0) return;
+  if (cp == null) return;
+
+  /** Total length of the input string. */
+  let len = input.length;
 
   let index = 0;
   let cursor = 0;
-  let cp = /** @type {number} */ (input.codePointAt(cursor));
 
   /** Category of codepoint immediately preceding cursor */
   let catBefore = cat(cp);