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Follow up from change 49c2427, copying the comment from the Java implementation to the C++ impl.

cpp/src/phonenumbers/asyoutypeformatter.cc

@@ -738,6 +738,8 @@ char AsYouTypeFormatter::NormalizeAndAccrueDigitsAndPlusSign(
 void AsYouTypeFormatter::InputDigitHelper(char next_char, string* number) {
   DCHECK(number);
   number->clear();
+  // Note that formattingTemplate is not guaranteed to have a value, it could be
+  // empty, e.g. when the next digit is entered after extracting an IDD or NDD.
   const char32 placeholder_codepoint = UnicodeString(kDigitPlaceholder)[0];
   int placeholder_pos = formatting_template_
       .tempSubString(last_match_position_).indexOf(placeholder_codepoint);

javascript/i18n/phonenumbers/asyoutypeformatter.js

@@ -1086,6 +1086,8 @@ i18n.phonenumbers.AsYouTypeFormatter.prototype.
 i18n.phonenumbers.AsYouTypeFormatter.prototype.inputDigitHelper_ =
     function(nextChar) {
 
+  // Note that formattingTemplate is not guaranteed to have a value, it could be
+  // empty, e.g. when the next digit is entered after extracting an IDD or NDD.
   /** @type {string} */
   var formattingTemplate = this.formattingTemplate_.toString();
   if (formattingTemplate.substring(this.lastMatchPosition_)