diff --git a/lib/rules_inline/state_inline.mjs b/lib/rules_inline/state_inline.mjs
index ddce556..225837b 100644
--- a/lib/rules_inline/state_inline.mjs
+++ b/lib/rules_inline/state_inline.mjs
@@ -120,6 +120,7 @@ StateInline.prototype.scanDelims = function (start, canSplitWord) {
     if (pos <= 0) { return 0x20 }
     const charCode = str.codePointAt(pos - 1)
     // not low surrogates (BMP)
+    // undefined & 0xFC00 = 0
     if ((charCode & 0xFC00) !== 0xDC00) { return charCode }
 
     // undefined if out of range (leading stray low surrogates)