diff --git a/src/LocateDependencies/LocateDependenciesViaComposer.php b/src/LocateDependencies/LocateDependenciesViaComposer.php
index e913d4ed..2ab1f3c1 100644
--- a/src/LocateDependencies/LocateDependenciesViaComposer.php
+++ b/src/LocateDependencies/LocateDependenciesViaComposer.php
@@ -56,8 +56,8 @@ public function __invoke(string $installationPath, bool $includeDevelopmentDepen
         }, $installationPath);
 
         return new AggregateSourceLocator([
-            (new MakeLocatorForInstalledJson())($installationPath, $this->astLocator),
             new PhpInternalSourceLocator($this->astLocator, new ReflectionSourceStubber()),
+            (new MakeLocatorForInstalledJson())($installationPath, $this->astLocator),
         ]);
     }
 
diff --git a/test/asset/composer-installation-with-vendor-overriding-internal-sources/composer.json b/test/asset/composer-installation-with-vendor-overriding-internal-sources/composer.json
new file mode 100644
index 00000000..c95f1dd3
--- /dev/null
+++ b/test/asset/composer-installation-with-vendor-overriding-internal-sources/composer.json
@@ -0,0 +1,3 @@
+{
+    "description": "a package"
+}
diff --git a/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/a/b/stringable-polyfill.php b/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/a/b/stringable-polyfill.php
new file mode 100644
index 00000000..81e1c5d5
--- /dev/null
+++ b/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/a/b/stringable-polyfill.php
@@ -0,0 +1,5 @@
+<?php
+
+interface Stringable
+{
+}
diff --git a/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/composer/installed.json b/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/composer/installed.json
new file mode 100644
index 00000000..93dddc32
--- /dev/null
+++ b/test/asset/composer-installation-with-vendor-overriding-internal-sources/vendor/composer/installed.json
@@ -0,0 +1,10 @@
+[
+  {
+    "name": "a/b",
+    "autoload": {
+      "files": [
+        "stringable-polyfill.php"
+      ]
+    }
+  }
+]
diff --git a/test/unit/LocateDependencies/LocateDependenciesViaComposerTest.php b/test/unit/LocateDependencies/LocateDependenciesViaComposerTest.php
index 8ab9a727..d853fae1 100644
--- a/test/unit/LocateDependencies/LocateDependenciesViaComposerTest.php
+++ b/test/unit/LocateDependencies/LocateDependenciesViaComposerTest.php
@@ -14,6 +14,7 @@
 use ReflectionProperty;
 use Roave\BackwardCompatibility\LocateDependencies\LocateDependenciesViaComposer;
 use Roave\BetterReflection\BetterReflection;
+use Roave\BetterReflection\Reflector\DefaultReflector;
 use Roave\BetterReflection\SourceLocator\Type\AggregateSourceLocator;
 use Roave\BetterReflection\SourceLocator\Type\PhpInternalSourceLocator;
 use Roave\BetterReflection\SourceLocator\Type\SourceLocator;
@@ -116,7 +117,20 @@ public function testWillLocateDependencies(): void
         ])->coerce($reflectionLocators->getValue($locator));
 
         self::assertCount(2, $locators);
-        self::assertInstanceOf(PhpInternalSourceLocator::class, $locators[1]);
+        self::assertInstanceOf(PhpInternalSourceLocator::class, $locators[0]);
+    }
+
+    public function testInternalReflectionStubsTakePriorityOverInstalledPolyfills(): void
+    {
+        $this->expectedInstallationPath = Type\string()
+            ->assert(Filesystem\canonicalize(__DIR__ . '/../../asset/composer-installation-with-vendor-overriding-internal-sources'));
+
+        $reflector = new DefaultReflector(($this->locateDependencies)($this->expectedInstallationPath, false));
+
+        self::assertTrue(
+            $reflector->reflectClass('Stringable')
+                ->isInternal()
+        );
     }
 
     public function testDevelopmentDependenciesCanBeOptionallyInstalled(): void
@@ -166,6 +180,6 @@ public function testDevelopmentDependenciesCanBeOptionallyInstalled(): void
         ])->coerce($reflectionLocators->getValue($locator));
 
         self::assertCount(2, $locators);
-        self::assertInstanceOf(PhpInternalSourceLocator::class, $locators[1]);
+        self::assertInstanceOf(PhpInternalSourceLocator::class, $locators[0]);
     }
 }