diff --git a/packages/faucet/src/faucet.ts b/packages/faucet/src/faucet.ts
index ed3c623d..b6450956 100644
--- a/packages/faucet/src/faucet.ts
+++ b/packages/faucet/src/faucet.ts
@@ -38,7 +38,7 @@ export class Faucet {
     apiUrl: string,
     addressPrefix: string,
     config: TokenConfiguration,
-    pens: ReadonlyArray<[string, Pen]>,
+    pens: ReadonlyArray<readonly [string, Pen]>,
     logging = false,
   ) {
     this.addressPrefix = addressPrefix;
diff --git a/packages/faucet/src/profile.ts b/packages/faucet/src/profile.ts
index dcd89358..0abc05a9 100644
--- a/packages/faucet/src/profile.ts
+++ b/packages/faucet/src/profile.ts
@@ -6,8 +6,8 @@ export async function createPens(
   addressPrefix: string,
   numberOfDistributors: number,
   logging = false,
-): Promise<ReadonlyArray<[string, Pen]>> {
-  const pens = new Array<[string, Pen]>();
+): Promise<ReadonlyArray<readonly [string, Pen]>> {
+  const pens = new Array<readonly [string, Pen]>();
 
   // first account is the token holder
   const numberOfIdentities = 1 + numberOfDistributors;