diff --git a/src/cjs/index.cjs b/src/cjs/index.cjs
index 07524c6..45c7330 100644
--- a/src/cjs/index.cjs
+++ b/src/cjs/index.cjs
@@ -82,8 +82,12 @@ function base (ALPHABET) {
     const b256 = new Uint8Array(size)
     // Process the characters.
     while (psz < source.length) {
+      // Find code of next character
+      const charCode = source.charCodeAt(psz)
+      // Base map can not be indexed using char code
+      if (charCode > 255) { return }
       // Decode character
-      let carry = BASE_MAP[source.charCodeAt(psz)]
+      let carry = BASE_MAP[charCode]
       // Invalid character
       if (carry === 255) { return }
       let i = 0
diff --git a/src/esm/index.js b/src/esm/index.js
index ce48a14..1593e17 100644
--- a/src/esm/index.js
+++ b/src/esm/index.js
@@ -80,8 +80,12 @@ function base (ALPHABET) {
     const b256 = new Uint8Array(size)
     // Process the characters.
     while (psz < source.length) {
+      // Find code of next character
+      const charCode = source.charCodeAt(psz)
+      // Base map can not be indexed using char code
+      if (charCode > 255) { return }
       // Decode character
-      let carry = BASE_MAP[source.charCodeAt(psz)]
+      let carry = BASE_MAP[charCode]
       // Invalid character
       if (carry === 255) { return }
       let i = 0
diff --git a/test/fixtures.json b/test/fixtures.json
index f8eedbf..3aadd67 100644
--- a/test/fixtures.json
+++ b/test/fixtures.json
@@ -660,6 +660,12 @@
       "alphabet": "0123456789fabcdef",
       "description": "poorly formed alphabet",
       "exception": "^TypeError: f is ambiguous$"
+    },
+    {
+      "alphabet": "base58",
+      "description": "character whose code exceeds the highest index of base map (>=256)",
+      "exception": "^Error: Non-base58 character$",
+      "string": "\u1000"
     }
   ]
 }
diff --git a/ts_src/index.ts b/ts_src/index.ts
index 5f2431a..9fa5c52 100644
--- a/ts_src/index.ts
+++ b/ts_src/index.ts
@@ -101,8 +101,14 @@ function base (ALPHABET: string): base.BaseConverter {
 
     // Process the characters.
     while (psz < source.length) {
+      // Find code of next character
+      const charCode = source.charCodeAt(psz)
+
+      // Base map can not be indexed using char code
+      if (charCode > 255) return
+
       // Decode character
-      let carry = BASE_MAP[source.charCodeAt(psz)]
+      let carry = BASE_MAP[charCode]
 
       // Invalid character
       if (carry === 255) return