diff --git a/lib/importmap/npm.rb b/lib/importmap/npm.rb
index b47f39e..4a54f85 100644
--- a/lib/importmap/npm.rb
+++ b/lib/importmap/npm.rb
@@ -86,7 +86,7 @@ def get_json(uri)
     end
 
     def find_latest_version(response)
-      latest_version = response.dig('dist-tags', 'latest')
+      latest_version = response.is_a?(String) ? response : response.dig('dist-tags', 'latest')
       return latest_version if latest_version
 
       return unless response['versions']
diff --git a/test/npm_test.rb b/test/npm_test.rb
index fd6f122..a53f8b0 100644
--- a/test/npm_test.rb
+++ b/test/npm_test.rb
@@ -94,4 +94,14 @@ def code() "200" end
       end
     end
   end
+
+  test "return latest version response is a String type" do
+    response = "version not found".to_json
+
+    @npm.stub(:get_json, response) do
+      outdated_packages = @npm.outdated_packages
+
+      assert_equal('version not found', outdated_packages[0].latest_version)
+    end
+  end
 end