diff --git a/src/Encoder/Encoder.php b/src/Encoder/Encoder.php
index c363953..6a7a951 100644
--- a/src/Encoder/Encoder.php
+++ b/src/Encoder/Encoder.php
@@ -25,6 +25,11 @@ final class Encoder
     /** @deprecated use DEFAULT_BYTE_MODE_ENCODING */
     public const DEFAULT_BYTE_MODE_ECODING = self::DEFAULT_BYTE_MODE_ENCODING;
 
+    /**
+     * Allowed characters for the Alphanumeric Mode.
+     */
+    private const ALPHANUMERIC_CHARS = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ $%*+-./:';
+
     /**
      * The original table is defined in the table 5 of JISX0510:2004 (p.19).
      */
@@ -148,13 +153,9 @@ public static function encode(
     /**
      * Gets the alphanumeric code for a byte.
      */
-    private static function getAlphanumericCode(int $code) : int
+    private static function getAlphanumericCode(int $byte) : int
     {
-        if (isset(self::ALPHANUMERIC_TABLE[$code])) {
-            return self::ALPHANUMERIC_TABLE[$code];
-        }
-
-        return -1;
+        return self::ALPHANUMERIC_TABLE[$byte] ?? -1;
     }
 
     /**
@@ -162,30 +163,20 @@ private static function getAlphanumericCode(int $code) : int
      */
     private static function chooseMode(string $content, ?string $encoding = null) : Mode
     {
+        if ('' === $content) {
+            return Mode::BYTE();
+        }
+
         if (null !== $encoding && 0 === strcasecmp($encoding, 'SHIFT-JIS')) {
             return self::isOnlyDoubleByteKanji($content) ? Mode::KANJI() : Mode::BYTE();
         }
 
-        $hasNumeric = false;
-        $hasAlphanumeric = false;
-        $contentLength = strlen($content);
-
-        for ($i = 0; $i < $contentLength; ++$i) {
-            $char = $content[$i];
-
-            if (ctype_digit($char)) {
-                $hasNumeric = true;
-            } elseif (-1 !== self::getAlphanumericCode(ord($char))) {
-                $hasAlphanumeric = true;
-            } else {
-                return Mode::BYTE();
-            }
+        if (ctype_digit($content)) {
+            return Mode::NUMERIC();
         }
 
-        if ($hasAlphanumeric) {
+        if (self::isOnlyAlphanumeric($content)) {
             return Mode::ALPHANUMERIC();
-        } elseif ($hasNumeric) {
-            return Mode::NUMERIC();
         }
 
         return Mode::BYTE();
@@ -205,7 +196,7 @@ private static function calculateMaskPenalty(ByteMatrix $matrix) : int
     }
 
     /**
-     * Checks if content only consists of double-byte kanji characters.
+     * Checks if content only consists of double-byte kanji characters (or is empty).
      */
     private static function isOnlyDoubleByteKanji(string $content) : bool
     {
@@ -232,6 +223,14 @@ private static function isOnlyDoubleByteKanji(string $content) : bool
         return true;
     }
 
+    /**
+     * Checks if content only consists of alphanumeric characters (or is empty).
+     */
+    private static function isOnlyAlphanumeric(string $content) : bool
+    {
+        return strlen($content) === strspn($content, self::ALPHANUMERIC_CHARS);
+    }
+
     /**
      * Chooses the best mask pattern for a matrix.
      */
diff --git a/test/Encoder/EncoderTest.php b/test/Encoder/EncoderTest.php
index 66459e9..bc90c7e 100644
--- a/test/Encoder/EncoderTest.php
+++ b/test/Encoder/EncoderTest.php
@@ -63,6 +63,10 @@ public function testGetAlphanumericCode() : void
 
     public function testChooseMode() : void
     {
+        // Empty string
+        $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, ''));
+        $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, '', 'SHIFT-JIS'));
+
         // Numeric mode
         $this->assertSame(Mode::NUMERIC(), $this->methods['chooseMode']->invoke(null, '0'));
         $this->assertSame(Mode::NUMERIC(), $this->methods['chooseMode']->invoke(null, '0123456789'));
@@ -77,7 +81,6 @@ public function testChooseMode() : void
         // 8-bit byte mode
         $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, 'a'));
         $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, '#'));
-        $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, ''));
 
         // AIUE in Hiragana in SHIFT-JIS
         $this->assertSame(Mode::BYTE(), $this->methods['chooseMode']->invoke(null, "\x8\xa\x8\xa\x8\xa\x8\xa6"));