diff --git a/libraries/deno-sshsig-mod.ts b/libraries/deno-sshsig-mod.ts
index 9c7574d..6693dd9 100644
--- a/libraries/deno-sshsig-mod.ts
+++ b/libraries/deno-sshsig-mod.ts
@@ -407,7 +407,7 @@ function parsePemToArray(pem: string): Uint8Array {
         }
     }
     if (innerPem.length === 0) {
-        return null;
+        return new Uint8Array(0);
     }
     return decodeBase64(innerPem.join(""));
 }
@@ -427,7 +427,9 @@ async function digestFile(filename: string, algorithm: string): Promise<Uint8Arr
     return new Uint8Array(hashBuffer);
 }
 
-function normalizeHashAlgorithm(algorithm: string): string {
+type DigestAlgorithm = "SHA-256" | "SHA-384" | "SHA-512";
+
+function normalizeHashAlgorithm(algorithm: string): DigestAlgorithm {
     let hashAlgorithm;
     switch (algorithm.toLowerCase()) {
         case "sha256":
@@ -445,7 +447,7 @@ function normalizeHashAlgorithm(algorithm: string): string {
         default:
             throw `Unknown algorithm: ${algorithm}`;
     }
-    return hashAlgorithm;
+    return hashAlgorithm as DigestAlgorithm;
 }
 
 async function testSshSig() {