diff --git a/ruby/lib/asciidoctor/extensions/asciidoctor_kroki/extension.rb b/ruby/lib/asciidoctor/extensions/asciidoctor_kroki/extension.rb
index ea839ca..4e41bba 100644
--- a/ruby/lib/asciidoctor/extensions/asciidoctor_kroki/extension.rb
+++ b/ruby/lib/asciidoctor/extensions/asciidoctor_kroki/extension.rb
@@ -294,7 +294,7 @@ def get_diagram_uri(server_url)
     end
 
     def encode
-      Base64.urlsafe_encode64(Zlib::Deflate.deflate(@text, 9))
+      ([Zlib::Deflate.deflate(@text, 9)].pack 'm0').tr '+/', '-_'
     end
 
     def save(output_dir_path, kroki_client)