diff --git a/.travis.yml b/.travis.yml
index addd889..028a0c7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,6 +16,10 @@ env:
     - RELEASE=stable COMPOSER_CHANNEL=snapshot
 
 matrix:
+  allow_failures:
+    # 5.6 is no longer supported by drupal-composer/drupal-project
+    - php: 5.6
+      env: RELEASE=stable COMPOSER_CHANNEL=stable
   exclude:
     - php: 5.6
       env: RELEASE=dev COMPOSER_CHANNEL=stable
diff --git a/src/Plugin.php b/src/Plugin.php
index 7069aac..51e474c 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -19,11 +19,14 @@ public function activate(Composer $composer, IOInterface $io)
         // Set default version constraints based on the composer requirements.
         $extra = $composer->getPackage()->getExtra();
         $packages = $composer->getPackage()->getRequires();
-        if (!isset($extra['composer-drupal-optimizations']['require']) && isset($packages['drupal/core'])) {
-            $coreConstraint = $packages['drupal/core']->getConstraint();
-            $extra['composer-drupal-optimizations']['require'] = static::getDefaultRequire($coreConstraint);
-            if (!empty($extra['composer-drupal-optimizations']['require']) && $io->isVerbose()) {
-                $io->write('Required tags were not explicitly set so the zaporylie/composer-drupal-optimizations set default based on project\'s composer.json content.');
+        if (!isset($extra['composer-drupal-optimizations']['require'])) {
+            $package = isset($packages['drupal/core']) ? $packages['drupal/core'] : (isset($packages['drupal/core-recommended']) ? $packages['drupal/core-recommended'] : null);
+            if (isset($package)) {
+                $coreConstraint = $package->getConstraint();
+                $extra['composer-drupal-optimizations']['require'] = static::getDefaultRequire($coreConstraint);
+                if (!empty($extra['composer-drupal-optimizations']['require']) && $io->isVerbose()) {
+                  $io->write('Required tags were not explicitly set so the zaporylie/composer-drupal-optimizations set default based on project\'s composer.json content.');
+                }
             }
         }
         if (!empty($extra['composer-drupal-optimizations']['require']) && $io->isVerbose()) {