diff --git a/src/index.js b/src/index.js
index 11e1a13..3f4bcab 100644
--- a/src/index.js
+++ b/src/index.js
@@ -80,7 +80,7 @@ function base (ALPHABET) {
     var size = (((source.length - psz) * FACTOR) + 1) >>> 0 // log(58) / log(256), rounded up.
     var b256 = new Uint8Array(size)
         // Process the characters.
-    while (source[psz]) {
+    while (psz < source.length) {
             // Decode character
       var carry = BASE_MAP[source.charCodeAt(psz)]
             // Invalid character
diff --git a/ts_src/index.ts b/ts_src/index.ts
index 2e71501..8c91160 100644
--- a/ts_src/index.ts
+++ b/ts_src/index.ts
@@ -99,7 +99,7 @@ function base (ALPHABET: string): base.BaseConverter {
     const b256 = new Uint8Array(size)
 
     // Process the characters.
-    while (source[psz]) {
+    while (psz < source.length) {
       // Decode character
       let carry = BASE_MAP[source.charCodeAt(psz)]