diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index ad2344187..8c8c62fab 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -111,25 +111,27 @@ class ValidatingArrayLoader implements LoaderInterface
             if (is_array($this->config['license']) || is_string($this->config['license'])) {
                 $licenses = (array) $this->config['license'];
 
-                // strip proprietary since it's not a valid SPDX identifier, but is accepted by composer
-                foreach ($licenses as $key => $license) {
-                    if ('proprietary' === $license) {
-                        unset($licenses[$key]);
-                    }
-                }
-
                 $licenseValidator = new SpdxLicenses();
-                if (count($licenses) === 1 && !$licenseValidator->validate($licenses) && $licenseValidator->validate(trim($licenses[0]))) {
-                    $this->warnings[] = sprintf(
-                        'License %s must not contain extra spaces, make sure to trim it.',
-                        json_encode($this->config['license'])
-                    );
-                } elseif (array() !== $licenses && !$licenseValidator->validate($licenses)) {
-                    $this->warnings[] = sprintf(
-                        'License %s is not a valid SPDX license identifier, see https://spdx.org/licenses/ if you use an open license.' . PHP_EOL .
-                        'If the software is closed-source, you may use "proprietary" as license.',
-                        json_encode($this->config['license'])
-                    );
+                foreach ($licenses as $license) {
+                    // replace proprietary by MIT for validation purposes since it's not a valid SPDX identifier, but is accepted by composer
+                    if ('proprietary' === $license) {
+                        continue;
+                    }
+                    $licenseToValidate = str_replace('proprietary', 'MIT', $license);
+                    if (!$licenseValidator->validate($licenseToValidate)) {
+                        if ($licenseValidator->validate(trim($licenseToValidate))) {
+                            $this->warnings[] = sprintf(
+                                'License %s must not contain extra spaces, make sure to trim it.',
+                                json_encode($license)
+                            );
+                        } else {
+                            $this->warnings[] = sprintf(
+                                'License %s is not a valid SPDX license identifier, see https://spdx.org/licenses/ if you use an open license.' . PHP_EOL .
+                                'If the software is closed-source, you may use "proprietary" as license.',
+                                json_encode($license)
+                            );
+                        }
+                    }
                 }
             }
         }