diff --git a/src/Config.php b/src/Config.php
index e6de5a5..89f79cd 100644
--- a/src/Config.php
+++ b/src/Config.php
@@ -4,6 +4,7 @@
 namespace Roots\WPConfig;
 
 use Roots\WPConfig\Exceptions\ConstantAlreadyDefinedException;
+use Roots\WPConfig\Exceptions\NumericConstantNameException;
 use Roots\WPConfig\Exceptions\UndefinedConfigKeyException;
 
 /**
@@ -24,6 +25,10 @@ class Config
      */
     public static function define(string $key, $value): void
     {
+        if (is_numeric($key)) {
+            throw new NumericConstantNameException("Numeric constant name '$key' is not allowed.");
+        }
+
         self::defined($key) or self::$configMap[$key] = $value;
     }
 
diff --git a/src/Exceptions/NumericConstantNameException.php b/src/Exceptions/NumericConstantNameException.php
new file mode 100644
index 0000000..4e5c431
--- /dev/null
+++ b/src/Exceptions/NumericConstantNameException.php
@@ -0,0 +1,9 @@
+<?php
+declare(strict_types=1);
+
+namespace Roots\WPConfig\Exceptions;
+
+class NumericConstantNameException extends \RuntimeException
+{
+
+}
diff --git a/tests/ConfigTest.php b/tests/ConfigTest.php
index 8a478c5..31a2e5f 100644
--- a/tests/ConfigTest.php
+++ b/tests/ConfigTest.php
@@ -5,6 +5,7 @@
 
 use PHPUnit\Framework\TestCase;
 use Roots\WPConfig\Exceptions\ConstantAlreadyDefinedException;
+use Roots\WPConfig\Exceptions\NumericConstantNameException;
 use Roots\WPConfig\Exceptions\UndefinedConfigKeyException;
 
 /**
@@ -17,6 +18,12 @@ public function testDefineHappy() {
     Config::define('WP_SCRIPT_DEBUG', true);
     $this->assertEquals(true, Config::get('WP_SCRIPT_DEBUG'));
   }
+
+  public function testDefineSadIntegerKey() {
+      $this->expectException(NumericConstantNameException::class);
+
+      Config::define('123', true);
+  }
   
   public function testDefineSad() {
     $this->expectException(ConstantAlreadyDefinedException::class);