diff --git a/acorn/src/tokenize.js b/acorn/src/tokenize.js
index a8d8f7a75..fdd69b472 100644
--- a/acorn/src/tokenize.js
+++ b/acorn/src/tokenize.js
@@ -316,7 +316,7 @@ pp.readToken_question = function() { // '?'
 
 pp.readToken_numberSign = function() { // '#'
   const ecmaVersion = this.options.ecmaVersion
-  let code = "#"
+  let code = 35 // '#'
   if (ecmaVersion >= 13) {
     ++this.pos
     code = this.fullCharCodeAtPos()
diff --git a/test/tests-class-features-2022.js b/test/tests-class-features-2022.js
index a74e0d75b..41e41e8dc 100644
--- a/test/tests-class-features-2022.js
+++ b/test/tests-class-features-2022.js
@@ -1755,6 +1755,9 @@ test("class C { #𩸽 }", {
   "sourceType": "script"
 }, {ecmaVersion: 13})
 
+// old ecma version
+testFail("class C { #aaa }", "Unexpected character '#' (1:10)", {ecmaVersion: 12})
+
 // Unexpected character
 testFail("class C { # aaa }", "Unexpected character ' ' (1:11)", {ecmaVersion: 13})
 testFail("class C { #+aaa }", "Unexpected character '+' (1:11)", {ecmaVersion: 13})