diff --git a/lib/datasource/docker/index.ts b/lib/datasource/docker/index.ts
index 4af7d2f4c2a843..f90cb7bb662c95 100644
--- a/lib/datasource/docker/index.ts
+++ b/lib/datasource/docker/index.ts
@@ -368,7 +368,7 @@ export async function getDigest(
     );
   }
   const cacheMinutes = 30;
-  await globalCache.set(cacheNamespace, cacheKey, null, cacheMinutes);
+  await globalCache.set(cacheNamespace, cacheKey, digest, cacheMinutes);
   return digest;
 }