diff --git a/source/dub/dub.d b/source/dub/dub.d
index 2822f1b..76636c6 100644
--- a/source/dub/dub.d
+++ b/source/dub/dub.d
@@ -504,7 +504,7 @@
 		This forwards to `Project.getDefaultConfiguration` and requires a
 		project to be loaded.
 	*/
-	string getDefaultConfiguration(BuildPlatform platform, bool allow_non_library_configs = true) const { return m_project.getDefaultConfiguration(platform, allow_non_library_configs); }
+	string getDefaultConfiguration(in BuildPlatform platform, bool allow_non_library_configs = true) const { return m_project.getDefaultConfiguration(platform, allow_non_library_configs); }
 
 	/** Attempts to upgrade the dependency selection of the loaded project.
 
diff --git a/source/dub/project.d b/source/dub/project.d
index b433b25..259a461 100644
--- a/source/dub/project.d
+++ b/source/dub/project.d
@@ -194,7 +194,7 @@
 				possible configuration instead of the first "executable"
 				configuration.
 	*/
-	string getDefaultConfiguration(BuildPlatform platform, bool allow_non_library_configs = true)
+	string getDefaultConfiguration(in BuildPlatform platform, bool allow_non_library_configs = true)
 	const {
 		auto cfgs = getPackageConfigs(platform, null, allow_non_library_configs);
 		return cfgs[m_rootPackage.name];