diff --git a/README.md b/README.md
index 6fea364..a0c60e1 100644
--- a/README.md
+++ b/README.md
@@ -145,7 +145,8 @@ export type PackageJsonLike = {
   peerDependencies?:     Record<string, string | undefined>;
 };
 
-export type InstalledDependencies = Map<string, PackageJsonLike>;
+// A map is allowed since that's what import('list-installed).listInstalled returns
+export type InstalledDependencies = Map<string, PackageJsonLike> | Record<string, PackageJsonLike>;
 ```
 
 #### Example
diff --git a/lib/check-package-versions.js b/lib/check-package-versions.js
index 4136be9..f4c312d 100644
--- a/lib/check-package-versions.js
+++ b/lib/check-package-versions.js
@@ -16,7 +16,7 @@ export function checkPackageVersions (mainPackage, installedDependencies) {
 
   for (const dependency of Object.keys(requiredDependencies)) {
     let targetVersion = requiredDependencies[dependency];
-    const installed = installedDependencies.get(dependency);
+    const installed = installedDependencies instanceof Map ? installedDependencies.get(dependency) : installedDependencies[dependency];
     const installedVersion = installed ? installed.version : undefined;
 
     if (!installed && !optionalDependencies[dependency]) {
diff --git a/lib/check-version-range.js b/lib/check-version-range.js
index 92574b4..b7e14a3 100644
--- a/lib/check-version-range.js
+++ b/lib/check-version-range.js
@@ -44,8 +44,8 @@ export function checkVersionRange (mainPackage, key, installedDependencies, opti
   /** @type {string | false} */
   let intersection = referenceRange;
 
-  for (const dependency of Object.keys(requiredDependencies)) {
-    const dependencyPackage = installedDependencies.get(dependency);
+  for (const dependency in requiredDependencies) {
+    const dependencyPackage = installedDependencies instanceof Map ? installedDependencies.get(dependency) : installedDependencies[dependency];
     const isOptional = !!optionalDependencies[dependency];
 
     const dependencyNotes = checkDependencyRange(referenceRange, key, dependencyPackage, isOptional, options);
@@ -79,11 +79,11 @@ export function checkVersionRange (mainPackage, key, installedDependencies, opti
  * @param {string} referenceRange
  * @param {string} key
  * @param {import('./get-installed-data.js').PackageJsonLike|undefined} dependencyPackage
- * @param {boolean} isOptional
+ * @param {boolean} [isOptional]
  * @param {Omit<VersionRangeOptions, 'noDev'>} [options]
- * @returns {VersionRangeItem | undefined}
+ * @returns {(VersionRangeItem & { valid: false | undefined }) | undefined}
  */
-function checkDependencyRange (referenceRange, key, dependencyPackage, isOptional, options) {
+export function checkDependencyRange (referenceRange, key, dependencyPackage, isOptional, options) {
   const {
     expectedInDependencies = false,
     strict = false,
diff --git a/lib/get-installed-data.js b/lib/get-installed-data.js
index a62122c..d30b369 100644
--- a/lib/get-installed-data.js
+++ b/lib/get-installed-data.js
@@ -13,7 +13,7 @@ import { readPackage } from 'read-pkg';
  * @property {Record<string, string | undefined>} [peerDependencies]
  */
 
-/** @typedef {Map<string, PackageJsonLike>} InstalledDependencies */
+/** @typedef {Map<string, PackageJsonLike> | Record<string, PackageJsonLike>} InstalledDependencies */
 
 /**
  * @typedef InstalledData