diff --git a/lib/ontologies_api_client/http.rb b/lib/ontologies_api_client/http.rb
index 9c7181f..6ec4d73 100644
--- a/lib/ontologies_api_client/http.rb
+++ b/lib/ontologies_api_client/http.rb
@@ -69,7 +69,7 @@ def self.get(path, params = {}, options = {})
               req.url path
               req.params = params.dup
               req.options[:timeout] = 60
-              req.headers.merge(headers)
+              req.headers.merge!(headers)
               req.headers[:invalidate_cache] = invalidate_cache
             end
           rescue StandardError => e