diff --git a/lib/cognito_idp/client.rb b/lib/cognito_idp/client.rb
index e27ae25..4e7b848 100644
--- a/lib/cognito_idp/client.rb
+++ b/lib/cognito_idp/client.rb
@@ -99,7 +99,7 @@ def basic_authorization_headers
       return if client_secret.nil?
 
       client_id_and_secret = "#{client_id}:#{client_secret}"
-      {"Authorization" => "Basic #{Base64.urlsafe_encode64(client_id_and_secret)}"}
+      {"Authorization" => "Basic #{Base64.strict_encode64(client_id_and_secret)}"}
     end
   end
 end
diff --git a/spec/cognito_idp/client_spec.rb b/spec/cognito_idp/client_spec.rb
index f2474e6..55b8405 100644
--- a/spec/cognito_idp/client_spec.rb
+++ b/spec/cognito_idp/client_spec.rb
@@ -102,7 +102,7 @@
           Faraday::Adapter::Test::Stubs.new do |stub|
             stub.post("https://auth.example.com/oauth2/token") do |env|
               id_and_secret = "#{client_id}:#{client_secret}"
-              basic_auth = "Basic #{Base64.urlsafe_encode64(id_and_secret)}"
+              basic_auth = "Basic #{Base64.strict_encode64(id_and_secret)}"
               fail "Basic Authorization is missing." unless env.request_headers["Authorization"] == basic_auth
               [200, {"Content-Type" => "application/json"}, response_payload.to_json]
             end
@@ -204,7 +204,7 @@
         Faraday::Adapter::Test::Stubs.new do |stub|
           stub.post("https://auth.example.com/oauth2/token", params_matcher) do |env|
             id_and_secret = "#{client_id}:#{client_secret}"
-            basic_auth = "Basic #{Base64.urlsafe_encode64(id_and_secret)}"
+            basic_auth = "Basic #{Base64.strict_encode64(id_and_secret)}"
             fail "Basic Authorization is missing." unless env.request_headers["Authorization"] == basic_auth
             [200, {"Content-Type" => "application/json"}, response_payload.to_json]
           end
@@ -325,7 +325,7 @@
           Faraday::Adapter::Test::Stubs.new do |stub|
             stub.post("https://auth.example.com/oauth2/token") do |env|
               id_and_secret = "#{client_id}:#{client_secret}"
-              basic_auth = "Basic #{Base64.urlsafe_encode64(id_and_secret)}"
+              basic_auth = "Basic #{Base64.strict_encode64(id_and_secret)}"
               fail "Basic Authorization is missing." unless env.request_headers["Authorization"] == basic_auth
               [200, {"Content-Type" => "application/json"}, response_payload.to_json]
             end
@@ -373,7 +373,7 @@
         Faraday::Adapter::Test::Stubs.new do |stub|
           stub.post("https://auth.example.com/oauth2/token", params_matcher) do |env|
             id_and_secret = "#{client_id}:#{client_secret}"
-            basic_auth = "Basic #{Base64.urlsafe_encode64(id_and_secret)}"
+            basic_auth = "Basic #{Base64.strict_encode64(id_and_secret)}"
             fail "Basic Authorization is missing." unless env.request_headers["Authorization"] == basic_auth
             [200, {"Content-Type" => "application/json"}, response_payload.to_json]
           end