diff --git a/scalar.go b/scalar.go
index c91574c..42bac75 100644
--- a/scalar.go
+++ b/scalar.go
@@ -74,6 +74,9 @@ type (
 // MarshalJSON implements the json.Marshaler interface.
 // The URI is a quoted string.
 func (u URI) MarshalJSON() ([]byte, error) {
+	if u.URL == nil {
+		return []byte("null"), nil
+	}
 	return json.Marshal(u.String())
 }
 
diff --git a/scalar_test.go b/scalar_test.go
index c2b34f0..c7bb450 100644
--- a/scalar_test.go
+++ b/scalar_test.go
@@ -16,6 +16,11 @@ func TestURI_MarshalJSON(t *testing.T) {
 		in   githubv4.URI
 		want string
 	}{
+		{
+			name: "null",
+			in:   githubv4.URI{},
+			want: `null`,
+		},
 		{
 			in:   githubv4.URI{URL: &url.URL{Scheme: "https", Host: "example.org", Path: "/foo/bar"}},
 			want: `"https://example.org/foo/bar"`,