diff --git a/client/client.go b/client/client.go
index 88c8333..77eb2d5 100644
--- a/client/client.go
+++ b/client/client.go
@@ -60,6 +60,8 @@
 
 	// ListIDs returns the IDs of all entries in the database.
 	ListIDs() ([]string, error)
+
+	unexported() // ensures that adding a method won't break users
 }
 
 type source interface {
@@ -71,6 +73,8 @@
 	dir string
 }
 
+func (*localSource) unexported() {}
+
 func (ls *localSource) GetByModule(module string) ([]*osv.Entry, error) {
 	content, err := ioutil.ReadFile(filepath.Join(ls.dir, module+".json"))
 	if os.IsNotExist(err) {
@@ -192,6 +196,8 @@
 	return index, nil
 }
 
+func (*httpSource) unexported() {}
+
 func (hs *httpSource) GetByModule(module string) ([]*osv.Entry, error) {
 	index, err := hs.Index()
 	if err != nil {
@@ -316,6 +322,8 @@
 	return c, nil
 }
 
+func (*client) unexported() {}
+
 func (c *client) GetByModule(module string) ([]*osv.Entry, error) {
 	var entries []*osv.Entry
 	// probably should be parallelized