diff --git a/src/Composer/Plugin.php b/src/Composer/Plugin.php
index 6554f9de0..c2bf0e833 100644
--- a/src/Composer/Plugin.php
+++ b/src/Composer/Plugin.php
@@ -68,10 +68,37 @@ public static function getSubscribedEvents() {
     return array(
       PackageEvents::POST_PACKAGE_INSTALL => "onPostPackageEvent",
       PackageEvents::POST_PACKAGE_UPDATE => "onPostPackageEvent",
+      ScriptEvents::PRE_INSTALL_CMD => 'checkInstallerPaths',
       ScriptEvents::POST_UPDATE_CMD => 'onPostCmdEvent',
     );
   }
 
+  /**
+   * Verify that composer.json contains correct values for installer-paths.
+   *
+   * Unfortunately, these values cannot be placed in composer.include.json.
+   *
+   * @see https://github.com/wikimedia/composer-merge-plugin/issues/139
+   *
+   * @param \Composer\Script\Event $event
+   */
+  public function checkInstallerPaths(Event $event) {
+    $extra = $this->composer->getPackage()->getExtra();
+    if (empty($extra['installer-paths'])) {
+      $this->io->write('<error>Error: extras.installer-paths is missing from your composer.json file.</error>');
+    }
+    else {
+      $composer_include_json_filename = $this->getVendorPath() . '/acquia/blt/template/composer.json';
+      if (file_exists($composer_include_json_filename)) {
+        $composer_include_json = json_decode(file_get_contents($composer_include_json_filename), TRUE);
+        if ($composer_include_json['extra']['installer-paths'] != $extra['installer-paths']) {
+          $this->io->write('<warning>Warning: The value for extras.installer-paths in composer.json differs from BLT\'s recommended values.</warning>');
+          $this->io->write('<warning>See https://github.com/acquia/blt/blob/8.x/template/composer.json</warning>');
+        }
+      }
+    }
+  }
+
   /**
    * Marks blt to be processed after an install or update command.
    *
@@ -191,7 +218,6 @@ public function getVendorPath() {
   protected function getOptions() {
     $defaults = [
       'update' => TRUE,
-      'composer-exclude-merge' => [],
     ];
     $extra = $this->composer->getPackage()->getExtra() + ['blt' => []];
     $extra['blt'] = $extra['blt'] + $defaults;