diff --git a/sshkit.gemspec b/sshkit.gemspec
index d42cad5b..1584bcfe 100644
--- a/sshkit.gemspec
+++ b/sshkit.gemspec
@@ -20,7 +20,7 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
   gem.version       = SSHKit::VERSION
 
-  gem.add_runtime_dependency('base64')
+  gem.add_runtime_dependency('base64') if RUBY_VERSION >= "2.4"
   gem.add_runtime_dependency('mutex_m')
   gem.add_runtime_dependency('net-ssh',  '>= 2.8.0')
   gem.add_runtime_dependency('net-scp',  '>= 1.1.2')