diff --git a/storage/filesystem.go b/storage/filesystem.go
index 7aac265..53f3882 100644
--- a/storage/filesystem.go
+++ b/storage/filesystem.go
@@ -235,8 +235,8 @@ func (b *filesystemBackend) AddressBook(ctx context.Context) (*carddav.AddressBo
 
 	debug.Printf("loading addressbook from %s", path)
 
-	data, err := ioutil.ReadFile(path)
-	if os.IsNotExist(err) {
+	data, readErr := ioutil.ReadFile(path)
+	if os.IsNotExist(readErr) {
 		urlPath, err := b.AddressbookHomeSetPath(ctx)
 		if err != nil {
 			return nil, err
@@ -246,9 +246,9 @@ func (b *filesystemBackend) AddressBook(ctx context.Context) (*carddav.AddressBo
 		if err != nil {
 			return nil, err
 		}
-		data, err = ioutil.ReadFile(path)
+		data, readErr = ioutil.ReadFile(path)
 	}
-	if err != nil {
+	if readErr != nil {
 		return nil, fmt.Errorf("error opening address book: %s", err.Error())
 	}
 	var addressBook carddav.AddressBook
@@ -492,7 +492,7 @@ func (b *filesystemBackend) Calendar(ctx context.Context) (*caldav.Calendar, err
 
 	debug.Printf("loading calendar from %s", path)
 
-	data, err := ioutil.ReadFile(path)
+	data, readErr := ioutil.ReadFile(path)
 	if os.IsNotExist(err) {
 		homeSetPath, err := b.CalendarHomeSetPath(ctx)
 		if err != nil {
@@ -503,9 +503,9 @@ func (b *filesystemBackend) Calendar(ctx context.Context) (*caldav.Calendar, err
 		if err != nil {
 			return nil, err
 		}
-		data, err = ioutil.ReadFile(path)
+		data, readErr = ioutil.ReadFile(path)
 	}
-	if err != nil {
+	if readErr != nil {
 		return nil, fmt.Errorf("error opening calendar: %s", err.Error())
 	}
 	var calendar caldav.Calendar