diff --git a/cmd/backup/config.go b/cmd/backup/config.go
index c696f9c..13f76c5 100644
--- a/cmd/backup/config.go
+++ b/cmd/backup/config.go
@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"os"
 	"regexp"
+	"strconv"
 	"time"
 )
 
@@ -75,7 +76,7 @@ type Config struct {
 	DropboxAppKey                 string          `split_words:"true"`
 	DropboxAppSecret              string          `split_words:"true"`
 	DropboxRemotePath             string          `split_words:"true"`
-	DropboxConcurrencyLevel       int             `split_words:"true" default:"6"`
+	DropboxConcurrencyLevel       NaturalNumber   `split_words:"true" default:"6"`
 }
 
 func (c *Config) resolveSecret(envVar string, secretPath string) (string, error) {
@@ -141,3 +142,21 @@ func (r *RegexpDecoder) Decode(v string) error {
 	*r = RegexpDecoder{Re: re}
 	return nil
 }
+
+type NaturalNumber int
+
+func (n *NaturalNumber) Decode(v string) error {
+	asInt, err := strconv.Atoi(v)
+	if err != nil {
+		return fmt.Errorf("config: error converting %s to int", v)
+	}
+	if asInt <= 0 {
+		return fmt.Errorf("config: expected a natural number, got %d", asInt)
+	}
+	*n = NaturalNumber(asInt)
+	return nil
+}
+
+func (n *NaturalNumber) Int() int {
+	return int(*n)
+}
diff --git a/cmd/backup/script.go b/cmd/backup/script.go
index cff52a0..1dace6c 100644
--- a/cmd/backup/script.go
+++ b/cmd/backup/script.go
@@ -227,7 +227,7 @@ func newScript() (*script, error) {
 			AppKey:           s.c.DropboxAppKey,
 			AppSecret:        s.c.DropboxAppSecret,
 			RemotePath:       s.c.DropboxRemotePath,
-			ConcurrencyLevel: s.c.DropboxConcurrencyLevel,
+			ConcurrencyLevel: s.c.DropboxConcurrencyLevel.Int(),
 		}
 		dropboxBackend, err := dropbox.NewStorageBackend(dropboxConfig, logFunc)
 		if err != nil {
diff --git a/internal/storage/dropbox/dropbox.go b/internal/storage/dropbox/dropbox.go
index f81637c..51b504b 100644
--- a/internal/storage/dropbox/dropbox.go
+++ b/internal/storage/dropbox/dropbox.go
@@ -8,7 +8,6 @@ import (
 	"os"
 	"path"
 	"path/filepath"
-	"reflect"
 	"strings"
 	"sync"
 	"time"
@@ -233,17 +232,18 @@ func (b *dropboxStorage) Prune(deadline time.Time, pruningPrefix string) (*stora
 	var matches []*files.FileMetadata
 	var lenCandidates int
 	for _, candidate := range entries {
-		if reflect.Indirect(reflect.ValueOf(candidate)).Type() != reflect.TypeOf(files.FileMetadata{}) {
+		switch candidate := candidate.(type) {
+		case *files.FileMetadata:
+			if !strings.HasPrefix(candidate.Name, pruningPrefix) {
+				continue
+			}
+			lenCandidates++
+			if candidate.ServerModified.Before(deadline) {
+				matches = append(matches, candidate)
+			}
+		default:
 			continue
 		}
-		candidate := candidate.(*files.FileMetadata)
-		if !strings.HasPrefix(candidate.Name, pruningPrefix) {
-			continue
-		}
-		lenCandidates++
-		if candidate.ServerModified.Before(deadline) {
-			matches = append(matches, candidate)
-		}
 	}
 
 	stats := &storage.PruneStats{
diff --git a/test/dropbox/docker-compose.yml b/test/dropbox/docker-compose.yml
index 748bbb4..a734098 100644
--- a/test/dropbox/docker-compose.yml
+++ b/test/dropbox/docker-compose.yml
@@ -5,9 +5,11 @@ services:
     image: muonsoft/openapi-mock
     environment:
       OPENAPI_MOCK_USE_EXAMPLES: if_present
-      OPENAPI_MOCK_SPECIFICATION_URL: https://raw.githubusercontent.com/offen/docker-volume-backup/2ea59a649be79d6cb22b8b5ad29e24377d4ab9d4/test/dropbox/user_v2.yaml
+      OPENAPI_MOCK_SPECIFICATION_URL: '/etc/openapi/user_v2.yaml'
     ports:
       - 8080:8080
+    volumes:
+      - ./user_v2.yaml:/etc/openapi/user_v2.yaml
 
   backup:
     image: offen/docker-volume-backup:${TEST_VERSION:-canary}
diff --git a/test/dropbox/openapi-mock.yaml b/test/dropbox/openapi-mock.yaml
deleted file mode 100644
index 21707b6..0000000
--- a/test/dropbox/openapi-mock.yaml
+++ /dev/null
@@ -1,25 +0,0 @@
-# OpenAPI specification options
-openapi:
-  specification_url: '/etc/openapi/user_v2.yaml'
-
-# web server options
-http:
-  cors_enabled: false
-  port: 8080
-  response_timeout: 1.0
-
-# application specific options
-application:
-  debug: false
-  log_format: tty
-  log_level: info
-
-# options to control generation process
-generation:
-  default_min_float: -1.073741823e+09
-  default_max_float: 1.073741823e+09
-  default_min_int: 0
-  default_max_int: 2147483647
-  null_probability: 0
-  suppress_errors: false
-  use_examples: 'if_present'