diff --git a/src/c14n-canonicalization.ts b/src/c14n-canonicalization.ts
index 7625217a..03406145 100644
--- a/src/c14n-canonicalization.ts
+++ b/src/c14n-canonicalization.ts
@@ -1,4 +1,4 @@
-import {
+import type {
   CanonicalizationOrTransformationAlgorithm,
   CanonicalizationOrTransformationAlgorithmProcessOptions,
   NamespacePrefix,
diff --git a/src/enveloped-signature.ts b/src/enveloped-signature.ts
index ab898bd4..80a0a553 100644
--- a/src/enveloped-signature.ts
+++ b/src/enveloped-signature.ts
@@ -1,6 +1,6 @@
 import * as xpath from "xpath";
 
-import {
+import type {
   CanonicalizationOrTransformationAlgorithm,
   CanonicalizationOrTransformationAlgorithmProcessOptions,
   CanonicalizationOrTransformAlgorithmType,
diff --git a/src/exclusive-canonicalization.ts b/src/exclusive-canonicalization.ts
index 06eea343..4f76c83a 100644
--- a/src/exclusive-canonicalization.ts
+++ b/src/exclusive-canonicalization.ts
@@ -1,4 +1,4 @@
-import {
+import type {
   CanonicalizationOrTransformationAlgorithm,
   CanonicalizationOrTransformationAlgorithmProcessOptions,
   NamespacePrefix,
diff --git a/src/hash-algorithms.ts b/src/hash-algorithms.ts
index 56f6c896..eeeb8b27 100644
--- a/src/hash-algorithms.ts
+++ b/src/hash-algorithms.ts
@@ -1,5 +1,5 @@
 import * as crypto from "crypto";
-import { HashAlgorithm } from "./types";
+import type { HashAlgorithm } from "./types";
 
 export class Sha1 implements HashAlgorithm {
   getHash = function (xml) {
diff --git a/src/signature-algorithms.ts b/src/signature-algorithms.ts
index 71a23ed8..46abe0eb 100644
--- a/src/signature-algorithms.ts
+++ b/src/signature-algorithms.ts
@@ -1,5 +1,5 @@
 import * as crypto from "crypto";
-import { SignatureAlgorithm, createOptionalCallbackFunction } from "./types";
+import { type SignatureAlgorithm, createOptionalCallbackFunction } from "./types";
 
 export class RsaSha1 implements SignatureAlgorithm {
   getSignature = createOptionalCallbackFunction(
diff --git a/src/signed-xml.ts b/src/signed-xml.ts
index b2c0ca0f..072201ff 100644
--- a/src/signed-xml.ts
+++ b/src/signed-xml.ts
@@ -1,4 +1,4 @@
-import {
+import type {
   CanonicalizationAlgorithmType,
   CanonicalizationOrTransformationAlgorithm,
   ComputeSignatureOptions,
diff --git a/src/utils.ts b/src/utils.ts
index e58a95d3..2bb860a8 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,5 +1,5 @@
 import * as xpath from "xpath";
-import { NamespacePrefix } from "./types";
+import type { NamespacePrefix } from "./types";
 
 export function isArrayHasLength(array: unknown): array is unknown[] {
   return Array.isArray(array) && array.length > 0;
@@ -207,6 +207,10 @@ function findNSPrefix(subset) {
   return subset.prefix || "";
 }
 
+function isElementSubset(docSubset: Node[]): docSubset is Element[] {
+  return docSubset.every((node) => xpath.isElement(node))
+}
+
 /**
  * Extract ancestor namespaces in order to import it to root of document subset
  * which is being canonicalized for non-exclusive c14n.
@@ -222,20 +226,17 @@ export function findAncestorNs(
   namespaceResolver?: XPathNSResolver
 ) {
   const docSubset = xpath.selectWithResolver(docSubsetXpath, doc, namespaceResolver);
-  let elementSubset: Element[] = [];
 
   if (!isArrayHasLength(docSubset)) {
     return [];
   }
 
-  if (!docSubset.every((node) => xpath.isElement(node))) {
+  if (!isElementSubset(docSubset)) {
     throw new Error("Document subset must be list of elements");
-  } else {
-    elementSubset = docSubset as Element[];
   }
 
   // Remove duplicate on ancestor namespace
-  const ancestorNs = collectAncestorNamespaces(elementSubset[0]);
+  const ancestorNs = collectAncestorNamespaces(docSubset[0]);
   const ancestorNsWithoutDuplicate: NamespacePrefix[] = [];
   for (let i = 0; i < ancestorNs.length; i++) {
     let notOnTheList = true;