diff --git a/lib/openssl/ssl.rb b/lib/openssl/ssl.rb
index 2186f5f43..245ddffe7 100644
--- a/lib/openssl/ssl.rb
+++ b/lib/openssl/ssl.rb
@@ -93,6 +93,7 @@ class SSLContext
       DEFAULT_CERT_STORE = OpenSSL::X509::Store.new # :nodoc:
       DEFAULT_CERT_STORE.set_default_paths
       DEFAULT_CERT_STORE.flags = OpenSSL::X509::V_FLAG_CRL_CHECK_ALL
+      DEFAULT_CERT_STORE.freeze
 
       # A callback invoked when DH parameters are required for ephemeral DH key
       # exchange.