diff --git a/storage/filesystem.go b/storage/filesystem.go
index bb5f2be..481f2d1 100644
--- a/storage/filesystem.go
+++ b/storage/filesystem.go
@@ -24,7 +24,7 @@ type filesystemBackend struct {
 
 var (
 	nilBackend         carddav.Backend = (*filesystemBackend)(nil)
-	validFilenameRegex                 = regexp.MustCompile(`^/[A-Za-z0-9_-]+(.[a-zA-Z]+)?$`)
+	validFilenameRegex                 = regexp.MustCompile(`^/[A-Za-z0-9][A-Za-z0-9_-]+(.[a-zA-Z]+)?$`)
 )
 
 func NewFilesystem(path string) (carddav.Backend, error) {
@@ -153,7 +153,7 @@ func (b *filesystemBackend) AddressBook(ctx context.Context) (*carddav.AddressBo
 	if err != nil {
 		return nil, err
 	}
-	path = filepath.Join(path, "default.json")
+	path = filepath.Join(path, "_default_ab.json")
 
 	data, err := ioutil.ReadFile(path)
 	if os.IsNotExist(err) {