Try explicitly loading arguments

config/services.yaml

@@ -14,3 +14,7 @@ services:
     Pcm\IconBundle\Twig\Runtime\IconRuntime:
         tags:
             - { name: twig.runtime }
+        arguments:
+            $defaultOptions: '%pcm.icon_bundle.default_options%'
+            $directories: '%pcm.icon_bundle.directories%'
+            $colours: '%pcm.icon_bundle.colours%'

src/DependencyInjection/PcmIconExtension.php

@@ -23,10 +23,8 @@ class PcmIconExtension extends Extension
         $configuration = new Configuration();
         $config = $this->processConfiguration($configuration, $configs);
 
-        $definition = $container->getDefinition('pcm_icon.icon_extension');
-
-        $definition->addArgument($config['default']);
-        $definition->addArgument($config['directories']);
-        $definition->addArgument($config['colours']);
+        $container->setParameter('pcm.icon_bundle.default_options', $config['default']);
+        $container->setParameter('pcm.icon_bundle.directories', $config['directories']);
+        $container->setParameter('pcm.icon_bundle.colours', $config['colours']);
     }
 }