diff --git a/src/interfaces.ts b/src/interfaces.ts
index d2ffb8844..cd5774837 100644
--- a/src/interfaces.ts
+++ b/src/interfaces.ts
@@ -116,7 +116,7 @@ export interface IPtyForkOptions {
   uid?: number;
   gid?: number;
   encoding?: string;
-  experimentalUseConpty?: boolean;
+  experimentalUseConpty?: boolean | undefined;
 }
 
 export interface IPtyOpenOptions {
diff --git a/typings/node-pty.d.ts b/typings/node-pty.d.ts
index 822a07444..30aa14b70 100644
--- a/typings/node-pty.d.ts
+++ b/typings/node-pty.d.ts
@@ -1,5 +1,6 @@
 /**
  * Copyright (c) 2017, Daniel Imms (MIT License).
+ * Copyright (c) 2018, Microsoft Corporation (MIT License).
  */
 
 declare module 'node-pty' {
@@ -26,8 +27,10 @@ declare module 'node-pty' {
     gid?: number;
     encoding?: string;
     /**
-     * Whether to use the experimental ConPTY system on Windows. This setting will be ignored on
-     * non-Windows.
+     * Whether to use the experimental ConPTY system on Windows. When this is not set, ConPTY will
+     * be used when the Windows build number is >= 17692.
+     *
+     * This setting does nothing on non-Windows.
      */
     experimentalUseConpty?: boolean;
   }