diff --git a/index.d.ts b/index.d.ts
index 4a82599..531c600 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -28,6 +28,9 @@ export declare class GPIOAccess extends EventEmitter {
      */
     unexportAll(): Promise<void>;
 }
+/**
+ * Different from Web GPIO API specification.
+ */
 export declare class GPIOPortMap extends Map<PortNumber, GPIOPort> {
 }
 export declare class GPIOPort extends EventEmitter {
diff --git a/index.js b/index.js
index f8e36c5..88da958 100644
--- a/index.js
+++ b/index.js
@@ -41,6 +41,9 @@ class GPIOAccess extends events_1.EventEmitter {
     }
 }
 exports.GPIOAccess = GPIOAccess;
+/**
+ * Different from Web GPIO API specification.
+ */
 class GPIOPortMap extends Map {
 }
 exports.GPIOPortMap = GPIOPortMap;
diff --git a/index.ts b/index.ts
index b601579..2e9c8eb 100644
--- a/index.ts
+++ b/index.ts
@@ -79,6 +79,9 @@ export class GPIOAccess extends EventEmitter {
   }
 }
 
+/**
+ * Different from Web GPIO API specification.
+ */
 export class GPIOPortMap extends Map<PortNumber, GPIOPort> {}
 
 export class GPIOPort extends EventEmitter {