diff --git a/tools/install-source/PathManglerFactory.cs b/tools/install-source/PathManglerFactory.cs
index c0cc5f2004..698daff0fb 100644
--- a/tools/install-source/PathManglerFactory.cs
+++ b/tools/install-source/PathManglerFactory.cs
@@ -82,6 +82,10 @@ namespace InstallSources
 					Console.WriteLine($"Mono path is {monoPath}");
 				return File.Exists(monoPath);
 			}
+			// check if the path is the xamarin source path + the mono external submodule
+			var monoSubmodule = Path.Combine (XamarinSourcePath.Replace ("src/", ""), "external", "mono");
+			if (path.StartsWith (monoSubmodule, StringComparison.Ordinal))
+				return true;
 			if (path.StartsWith (XamarinSourcePath, StringComparison.Ordinal))
 				return false;
 			var xamarinRuntimePath = XamarinSourcePath.Replace($"/{srcSubPath}/", $"/{runtimeSubPath}/");