diff --git a/Plugins/SelectiveTestingPlugin/SelectiveTestingPlugin.swift b/Plugins/SelectiveTestingPlugin/SelectiveTestingPlugin.swift
index c282dbb..1b0eed2 100644
--- a/Plugins/SelectiveTestingPlugin/SelectiveTestingPlugin.swift
+++ b/Plugins/SelectiveTestingPlugin/SelectiveTestingPlugin.swift
@@ -56,6 +56,6 @@ struct SelectiveTestingPlugin: CommandPlugin {
     }
 #endif
 
-extension String: LocalizedError {
+extension String: @retroactive LocalizedError {
     public var errorDescription: String? { return self }
 }
diff --git a/Sources/SelectiveTestShell/String+Error.swift b/Sources/SelectiveTestShell/String+Error.swift
index c4180ab..abbfcf1 100644
--- a/Sources/SelectiveTestShell/String+Error.swift
+++ b/Sources/SelectiveTestShell/String+Error.swift
@@ -4,4 +4,8 @@
 
 import Foundation
 
+#if compiler(>=6)
+extension String: @retroactive Error {}
+#else
 extension String: Error {}
+#endif
diff --git a/Sources/Workspace/XCWorkspace+Projects.swift b/Sources/Workspace/XCWorkspace+Projects.swift
index 512ed9e..3ee5c35 100644
--- a/Sources/Workspace/XCWorkspace+Projects.swift
+++ b/Sources/Workspace/XCWorkspace+Projects.swift
@@ -6,7 +6,11 @@ import Foundation
 import PathKit
 import XcodeProj
 
+#if compiler(>=6)
+extension String: @retroactive Error {}
+#else
 extension String: Error {}
+#endif
 
 extension XCWorkspace {
     public func allProjects(basePath: Path) throws -> [(XcodeProj, Path)] {