diff --git a/src/services/services.ts b/src/services/services.ts
index dcc321b00d5..e9af862d002 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1217,6 +1217,10 @@ namespace ts {
                 getDirectories: path => {
                     return host.getDirectories ? host.getDirectories(path) : [];
                 },
+                readDirectory(path, extensions, exclude, include, depth) {
+                    Debug.assertDefined(host.readDirectory, "'LanguageServiceHost.readDirectory' must be implemented to correctly process 'projectReferences'");
+                    return host.readDirectory!(path, extensions, exclude, include, depth);
+                },
                 onReleaseOldSourceFile,
                 hasInvalidatedResolution,
                 hasChangedAutomaticTypeDirectiveNames: host.hasChangedAutomaticTypeDirectiveNames