diff --git a/src/Composer/Package/Loader/RootPackageLoader.php b/src/Composer/Package/Loader/RootPackageLoader.php
index 2115d7378..32118b113 100644
--- a/src/Composer/Package/Loader/RootPackageLoader.php
+++ b/src/Composer/Package/Loader/RootPackageLoader.php
@@ -190,6 +190,8 @@ class RootPackageLoader extends ArrayLoader
                     'alias' => $match[2],
                     'alias_normalized' => $this->versionParser->normalize($match[2], $reqVersion),
                 );
+            } elseif (strpos($reqVersion, ' as ') !== false) {
+                throw new \UnexpectedValueException('Invalid alias definition in "'.$reqName.'": "'.$reqVersion.'". Aliases should be in the form "exact-version as other-exact-version".');
             }
         }