diff --git a/src/index.ts b/src/index.ts
index 885b0b819..3f3d10d6c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -489,7 +489,7 @@ export function create (rawOptions: CreateOptions = {}): Register {
           return ts.ScriptSnapshot.fromString(contents)
         },
         readFile: cachedReadFile,
-        readDirectory: cachedLookup(debugFn('readDirectory', ts.sys.readDirectory)),
+        readDirectory: ts.sys.readDirectory,
         getDirectories: cachedLookup(debugFn('getDirectories', ts.sys.getDirectories)),
         fileExists: cachedLookup(debugFn('fileExists', fileExists)),
         directoryExists: cachedLookup(debugFn('directoryExists', ts.sys.directoryExists)),