diff --git a/src/us/ssn.ts b/src/us/ssn.ts
index a93c76ea..d327bab4 100644
--- a/src/us/ssn.ts
+++ b/src/us/ssn.ts
@@ -17,7 +17,7 @@ import * as exceptions from '../exceptions';
 import { strings } from '../util';
 import { Validator, ValidateReturn } from '../types';
 
-const invalidSSN = [
+const invalidSSNSet = new Set([
   '111111111',
   '222222222',
   '333333333',
@@ -27,7 +27,6 @@ const invalidSSN = [
   '888888888',
   '999999999',
   '123123123',
-  '999999999',
   // Used in Advertising and known "invalid"
   '002281852',
   '042103580',
@@ -51,7 +50,7 @@ const invalidSSN = [
   '457555462',
   '468288779',
   '549241889',
-];
+]);
 
 function clean(input: string): ReturnType<typeof strings.cleanUnicode> {
   return strings.cleanUnicode(input, '- ');
@@ -94,7 +93,7 @@ const impl: Validator = {
     if (!strings.isdigits(value)) {
       return { isValid: false, error: new exceptions.InvalidComponent() };
     }
-    if (invalidSSN.includes(value)) {
+    if (invalidSSNSet.has(value)) {
       return { isValid: false, error: new exceptions.InvalidComponent() };
     }
     if (/^(000|666|9)\d+/.test(value)) {
diff --git a/src/util/checksum.ts b/src/util/checksum.ts
index 0e76a469..3f0d4263 100644
--- a/src/util/checksum.ts
+++ b/src/util/checksum.ts
@@ -80,7 +80,6 @@ export function luhnChecksumValidate(
 
   const sum = value
     .split('')
-    // .reverse()
     .map(v => alphabet.indexOf(v))
     .reduce((acc, val, idx) => {
       let v = val;
@@ -213,6 +212,25 @@ function modulo(dividentIn: string, divisor: number) {
   return parseInt(divident, 10) % divisor;
 }
 
+/**
+ * Convert a string using alphanumeric alphabet to its numeric representation.
+ * Returns null if any character is not in the alphabet.
+ */
+function alphabetToNumber(
+  value: string,
+  alphabet = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ',
+): string | null {
+  let result = '';
+  for (const c of value) {
+    const idx = alphabet.indexOf(c);
+    if (idx === -1) {
+      return null;
+    }
+    result += String(idx);
+  }
+  return result;
+}
+
 /**
  * The ISO 7064 Mod 97, 10 algorithm.
  *
@@ -220,25 +238,10 @@ function modulo(dividentIn: string, divisor: number) {
  * valid if the number modulo 97 is 1. As such it has two check digits.
  */
 export function mod97base10Validate(value: string, expect = 1): boolean {
-  const alphabet = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
-  let fail = false;
-
-  const bigValue = value
-    .split('')
-    .map(c => {
-      const idx = alphabet.indexOf(c);
-      if (idx === -1) {
-        fail = true;
-        return '';
-      }
-      return String(idx);
-    })
-    .join('');
-
-  if (fail) {
+  const bigValue = alphabetToNumber(value);
+  if (bigValue === null) {
     return false;
   }
-
   return modulo(bigValue, 97) === expect;
 }