diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4568b81..8f95b13 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 ### Unreleased
 
+### v1.5.0-beta3 (2020-10-09)
+
+* Init the DeviceIdentifier to a fixed value in the CLI environment without setting any cookies, to prevent
+  errors if the process has already sent output.
 * Add MutexWrapper with Mock and Db (mysql) backed implementations for preventing concurrent executions
   of code.
  
diff --git a/src/Logging/DeviceIdentifier.php b/src/Logging/DeviceIdentifier.php
index e3c5838..7c8667d 100644
--- a/src/Logging/DeviceIdentifier.php
+++ b/src/Logging/DeviceIdentifier.php
@@ -45,10 +45,19 @@ public static function forceGlobalTestValue(string $id): void
      * before sending any output.
      *
      * @param bool $ssl_available
+     * @param bool $is_cli
      */
-    public static function initAndEnsureCookieSet(bool $ssl_available): void
+    public static function initAndEnsureCookieSet(bool $ssl_available, bool $is_cli = (PHP_SAPI === 'cli')): void
     {
-        static::$instance = new DeviceIdentifier(new CookieWrapper($ssl_available));
+        if ($is_cli) {
+            // Stub the class without setting cookies for use in a CLI environment.
+            // Note that as it's not really appropriate to inject the `is_cli` arg to `::get()`, and we can't stub
+            // PHP_SAPI for testing, that method will still return `-unset-` if it is called before
+            // ::initAndEnsureCookieSet()
+            static::forceGlobalTestValue(static::CLI_ID);
+        } else {
+            static::$instance = new DeviceIdentifier(new CookieWrapper($ssl_available));
+        }
         static::$instance->init();
     }
 
@@ -68,8 +77,9 @@ public static function get(): string
         return $i->getValue();
     }
 
+    const CLI_ID          = 'cli-------------------';
     const COOKIE_LIFETIME = 'P5Y';
-    const VALID_REGEX = '/^[a-zA-Z0-9\-_=]{22}$/';
+    const VALID_REGEX     = '/^[a-zA-Z0-9\-_=]{22}$/';
 
     /**
      * @var string|null
diff --git a/test/unit/Logging/DeviceIdentifierTest.php b/test/unit/Logging/DeviceIdentifierTest.php
index 517a1e3..6fa56c3 100644
--- a/test/unit/Logging/DeviceIdentifierTest.php
+++ b/test/unit/Logging/DeviceIdentifierTest.php
@@ -154,7 +154,7 @@ function () { DeviceIdentifier::$instance = NULL; }
         $old_cookie = $_COOKIE;
         try {
             $_COOKIE['did'] = '1234567890123456789012';
-            DeviceIdentifier::initAndEnsureCookieSet(TRUE);
+            DeviceIdentifier::initAndEnsureCookieSet(TRUE, FALSE);
             $this->assertSame('1234567890123456789012', DeviceIdentifier::get());
 
             // Note that here the init method has assigned a global instance so changes to $_COOKIE
@@ -168,6 +168,18 @@ function () { DeviceIdentifier::$instance = NULL; }
         $this->assertSame('1234567890123456789012', DeviceIdentifier::get());
     }
 
+    public function test_its_static_init_creates_instance_and_initialises_with_fixed_id_in_cli()
+    {
+        ScopeChangingCaller::call(
+            DeviceIdentifier::class,
+            function () { DeviceIdentifier::$instance = NULL; }
+        );
+
+        DeviceIdentifier::initAndEnsureCookieSet(TRUE); // Fall back to default arg
+        $this->assertSame(DeviceIdentifier::CLI_ID, DeviceIdentifier::get());
+        $this->assertRegExp(DeviceIdentifier::VALID_REGEX, DeviceIdentifier::get());
+    }
+
     public function test_its_static_get_can_read_from_cookies_even_if_not_initialised()
     {
         ScopeChangingCaller::call(