diff --git a/src/caoscrawler/crawl.py b/src/caoscrawler/crawl.py
index ccad944776fb51fef6ed566b999340f06f838705..06059ed1eb532948419bfadd98473333a108203e 100644
--- a/src/caoscrawler/crawl.py
+++ b/src/caoscrawler/crawl.py
@@ -1088,6 +1088,8 @@ def crawler_main(crawled_directory_path: str,
             if isinstance(elem, db.File):
                 # correct the file path:
                 # elem.file = os.path.join(args.path, elem.file)
+                if prefix is None:
+                    raise RuntimeError("No prefix set. Prefix must be set if files are used.")
                 if elem.path.startswith(prefix):
                     elem.path = elem.path[len(prefix):]
                 elem.file = None