diff --git a/.rubocop.yml b/.rubocop.yml
index d9fb038..67285fc 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -7,9 +7,7 @@ AllCops:
     - "Rakefile"
     - "rakelib/**/*"
 Metrics/ClassLength:
-  Max: 110
-  Exclude:
-    - "lib/googleauth/credentials.rb"
+  Max: 200
 Metrics/ModuleLength:
   Max: 110
 Metrics/BlockLength:
diff --git a/lib/googleauth/user_authorizer.rb b/lib/googleauth/user_authorizer.rb
index 59f4647..90c3761 100644
--- a/lib/googleauth/user_authorizer.rb
+++ b/lib/googleauth/user_authorizer.rb
@@ -271,10 +271,15 @@ module Google
       # @return [String]
       #  Redirect URI
       def redirect_uri_for base_url
-        return @callback_uri unless URI(@callback_uri).scheme.nil?
+        return @callback_uri if uri_is_postmessage?(@callback_uri) || !URI(@callback_uri).scheme.nil?
         raise format(MISSING_ABSOLUTE_URL_ERROR, @callback_uri) if base_url.nil? || URI(base_url).scheme.nil?
         URI.join(base_url, @callback_uri).to_s
       end
+
+      # Check if URI is Google's postmessage flow (not a valid redirect_uri by spec, but allowed)
+      def uri_is_postmessage? uri
+        uri.to_s.casecmp("postmessage").zero?
+      end
     end
   end
 end
diff --git a/spec/googleauth/user_authorizer_spec.rb b/spec/googleauth/user_authorizer_spec.rb
index 5a36304..50f3f2f 100644
--- a/spec/googleauth/user_authorizer_spec.rb
+++ b/spec/googleauth/user_authorizer_spec.rb
@@ -80,7 +80,7 @@ describe Google::Auth::UserAuthorizer do
       expect(URI(uri).query).to_not match(/client_secret/)
     end
 
-    it "should include the callback uri" do
+    it "should include the redirect_uri" do
       expect(URI(uri).query).to match(
         %r{redirect_uri=https://www.example.com/oauth/callback}
       )
@@ -91,6 +91,25 @@ describe Google::Auth::UserAuthorizer do
     end
   end
 
+  context "when generating authorization URLs and callback_uri is 'postmessage'" do
+    let(:callback_uri) { "postmessage" }
+    let :authorizer do
+      Google::Auth::UserAuthorizer.new(client_id,
+                                       scope,
+                                       token_store,
+                                       callback_uri)
+    end
+    let :uri do
+      authorizer.get_authorization_url login_hint: "user1", state: "mystate"
+    end
+
+    it "should include the redirect_uri 'postmessage'" do
+      expect(URI(uri).query).to match(
+        %r{redirect_uri=postmessage}
+      )
+    end
+  end
+
   context "when generating authorization URLs with user ID & state" do
     let :uri do
       authorizer.get_authorization_url login_hint: "user1", state: "mystate"
@@ -253,6 +272,7 @@ describe Google::Auth::UserAuthorizer do
         user_id: "user1", code: "code"
       )
       expect(credentials.access_token).to eq "1/abc123"
+      expect(credentials.redirect_uri.to_s).to eq "https://www.example.com/oauth/callback"
     end
 
     it "should not store credentials when get only requested" do