diff --git a/mod.ts b/mod.ts
index 055aeef..d1a5d99 100644
--- a/mod.ts
+++ b/mod.ts
@@ -6,6 +6,7 @@ import * as path from "https://deno.land/std@0.185.0/path/mod.ts";
 import * as cookie from "https://deno.land/std@0.185.0/http/cookie.ts";
 import { Aes } from "https://deno.land/x/crypto/aes.ts";
 import { Cbc, Padding } from "https://deno.land/x/crypto/block-modes.ts";
+import { cryptoRandomString } from "https://deno.land/x/crypto_random_string@1.0.0/mod.ts";
 
 type HTTPServerOptions = {
   port: number;
@@ -61,6 +62,12 @@ export class HTTPServer {
   settings?: HTTPServerOptions;
 
   async listen(options: HTTPServerOptions) {
+    if (options.sessionSecret) {
+      if (![16, 24, 32].includes(options.sessionSecret.length)) {
+        const randomString = cryptoRandomString({ length: 32 });
+        throw new Error("\nInvalid key size (must be either 16, 24 or 32 bytes)\nHere is a pregenerated key: " + randomString);
+      }
+    }
     this.settings = options;
     this.server = Deno.listen({
       port: options.port,
@@ -286,7 +293,7 @@ export class HTTPServer {
         routeReply.cookie("session", encodedSession, {
           maxAge: this.settings.sessionExpire ?? undefined,
         });
-      }else{
+      } else {
         routeReply.cookie("session", undefined);
       }
     }