diff --git a/lib/manager/npm/post-update/index.js b/lib/manager/npm/post-update/index.js
index 840cc5e3dc31b74d4eb4a2aec0bbd0fe0d342f9e..62a4bda30955cf83c9743e5e5dedb8d18090659b 100644
--- a/lib/manager/npm/post-update/index.js
+++ b/lib/manager/npm/post-update/index.js
@@ -2,6 +2,7 @@ const fs = require('fs-extra');
 const path = require('path');
 const upath = require('upath');
 const os = require('os');
+const is = require('@sindresorhus/is');
 
 const npm = require('./npm');
 const lerna = require('./lerna');
@@ -266,7 +267,7 @@ function listLocalLibs(dependencies) {
   const toCopy = [];
   if (dependencies) {
     for (const [libName, libVersion] of Object.entries(dependencies)) {
-      if (libVersion.startsWith('file:')) {
+      if (is.string(libVersion) && libVersion.startsWith('file:')) {
         if (libVersion.endsWith('.tgz')) {
           logger.info(
             `Link to local lib "${libName}": "${libVersion}" is not supported. Please do it like: 'file:/path/to/folder'`