diff --git a/lib/manager/npm/extract.js b/lib/manager/npm/extract.js
index 6d9db6c5b151a468198ebeb7668c3cb9282a82b5..1f01c9d4e76f239bfc5e688b7dcd516a8bac3cea 100644
--- a/lib/manager/npm/extract.js
+++ b/lib/manager/npm/extract.js
@@ -15,7 +15,7 @@ function extractDependencies(packageJson, config) {
   return depNames
     .map(depName => {
       const currentVersion = packageJson[depType][depName]
-        ? packageJson[depType][depName].trim().replace(/^=/, '')
+        ? `${packageJson[depType][depName]}`.trim().replace(/^=/, '')
         : undefined;
       let lockedVersion;
       try {