diff --git a/index.js b/index.js
index 88da958..b24c991 100644
--- a/index.js
+++ b/index.js
@@ -8,7 +8,7 @@ const path = require("path");
  */
 const PollingInterval = 100;
 const SysfsGPIOPath = "/sys/class/gpio";
-const GPIOPortMapSizeMax = 1023;
+const GPIOPortMapSizeMax = 1024;
 const Uint16Max = 65535;
 function parseUint16(string) {
     const n = Number.parseInt(string, 10);
@@ -154,7 +154,7 @@ class OperationError extends Error {
 }
 exports.OperationError = OperationError;
 async function requestGPIOAccess() {
-    const ports = new GPIOPortMap([...Array(GPIOPortMapSizeMax + 1).keys()].map(portNumber => [
+    const ports = new GPIOPortMap([...Array(GPIOPortMapSizeMax).keys()].map(portNumber => [
         portNumber,
         new GPIOPort(portNumber)
     ]));
diff --git a/index.ts b/index.ts
index 2e9c8eb..1740487 100644
--- a/index.ts
+++ b/index.ts
@@ -9,7 +9,7 @@ const PollingInterval = 100;
 
 const SysfsGPIOPath = "/sys/class/gpio";
 
-const GPIOPortMapSizeMax = 1023;
+const GPIOPortMapSizeMax = 1024;
 
 const Uint16Max = 65535;
 
@@ -232,7 +232,7 @@ export class OperationError extends Error {}
 
 export async function requestGPIOAccess(): Promise<GPIOAccess> {
   const ports = new GPIOPortMap(
-    [...Array(GPIOPortMapSizeMax + 1).keys()].map(portNumber => [
+    [...Array(GPIOPortMapSizeMax).keys()].map(portNumber => [
       portNumber,
       new GPIOPort(portNumber)
     ])