diff --git a/lib/modules/manager/npm/extract/monorepo.ts b/lib/modules/manager/npm/extract/monorepo.ts
index ea5db558a15e7c3070cb4df33b688b647c7c39b0..2f3c2cc3c4a54453b7ba2a1a0bd9bb1d8278ad3a 100644
--- a/lib/modules/manager/npm/extract/monorepo.ts
+++ b/lib/modules/manager/npm/extract/monorepo.ts
@@ -42,7 +42,10 @@ export async function detectMonorepos(
         .filter(Boolean);
 
       p.deps?.forEach((dep) => {
-        if (internalPackageNames.includes(dep.depName)) {
+        if (
+          is.string(dep.depName) &&
+          internalPackageNames.includes(dep.depName)
+        ) {
           dep.isInternal = true;
         }
       });