diff --git a/src/Composer/Factory.php b/src/Composer/Factory.php
index 8b7c4dbdb..3d6d58318 100644
--- a/src/Composer/Factory.php
+++ b/src/Composer/Factory.php
@@ -223,7 +223,19 @@ class Factory
 
     public static function getComposerFile(): string
     {
-        return trim((string) Platform::getEnv('COMPOSER')) ?: './composer.json';
+        $env = Platform::getEnv('COMPOSER');
+        if (is_string($env)) {
+            $env = trim($env);
+            if ('' !== $env) {
+                if (is_dir($env)) {
+                    throw new \RuntimeException('The COMPOSER environment variable is set to '.$env.' which is a directory, this variable should point to a composer.json or be left unset.');
+                }
+
+                return $env;
+            }
+        }
+
+        return './composer.json';
     }
 
     public static function getLockFile(string $composerFile): string
diff --git a/tests/Composer/Test/FactoryTest.php b/tests/Composer/Test/FactoryTest.php
index 636e58c31..a4b5e6df1 100644
--- a/tests/Composer/Test/FactoryTest.php
+++ b/tests/Composer/Test/FactoryTest.php
@@ -13,9 +13,16 @@
 namespace Composer\Test;
 
 use Composer\Factory;
+use Composer\Util\Platform;
 
 class FactoryTest extends TestCase
 {
+    public function tearDown(): void
+    {
+        parent::tearDown();
+        Platform::clearEnv('COMPOSER');
+    }
+
     /**
      * @group TLS
      */
@@ -37,4 +44,23 @@ class FactoryTest extends TestCase
 
         Factory::createHttpDownloader($ioMock, $config);
     }
+
+    public function testGetComposerJsonPath(): void
+    {
+        self::assertSame('./composer.json', Factory::getComposerFile());
+    }
+
+    public function testGetComposerJsonPathFailsIfDir(): void
+    {
+        Platform::putEnv('COMPOSER', __DIR__);
+        self::expectException('RuntimeException');
+        self::expectExceptionMessage('The COMPOSER environment variable is set to '.__DIR__.' which is a directory, this variable should point to a composer.json or be left unset.');
+        Factory::getComposerFile();
+    }
+
+    public function testGetComposerJsonPathFromEnv(): void
+    {
+        Platform::putEnv('COMPOSER', ' foo.json ');
+        self::assertSame('foo.json', Factory::getComposerFile());
+    }
 }