diff --git a/src/Composer/Package/Loader/ArrayLoader.php b/src/Composer/Package/Loader/ArrayLoader.php
index 3d315045d965..9a042d9f224c 100644
--- a/src/Composer/Package/Loader/ArrayLoader.php
+++ b/src/Composer/Package/Loader/ArrayLoader.php
@@ -421,6 +421,8 @@ public function getBranchAlias(array $config): ?string
 
         if (isset($config['extra']['branch-alias']) && \is_array($config['extra']['branch-alias'])) {
             foreach ($config['extra']['branch-alias'] as $sourceBranch => $targetBranch) {
+                $sourceBranch = (string) $sourceBranch;
+
                 // ensure it is an alias to a -dev package
                 if ('-dev' !== substr($targetBranch, -4)) {
                     continue;
diff --git a/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php b/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
index 2fb83698b9c5..a3f5d28c6e40 100644
--- a/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
+++ b/tests/Composer/Test/Package/Loader/ArrayLoaderTest.php
@@ -347,4 +347,23 @@ public function testNoneStringSourceDistReference(): void
         $this->assertSame('2019', $package->getSourceReference());
         $this->assertSame('2019', $package->getDistReference());
     }
+
+    public function testBranchAliasIntegerIndex(): void
+    {
+        $config = array(
+            'name' => 'acme/package',
+            'version' => 'dev-1',
+            'extra' => [
+                'branch-alias' => [
+                    '1' => '1.3-dev',
+                ],
+            ],
+            'dist' => [
+                'type' => 'zip',
+                'url' => 'https://example.org/',
+            ],
+        );
+
+        $this->assertNull($this->loader->getBranchAlias($config));
+    }
 }