diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index c3c95522..2cd8f9bb 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "1.9.0"
+  ".": "1.9.1"
 }
\ No newline at end of file
diff --git a/.stats.yml b/.stats.yml
index 33679484..a3a2c778 100644
--- a/.stats.yml
+++ b/.stats.yml
@@ -1,4 +1,4 @@
 configured_endpoints: 30
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/open-transit/open-transit-5f69b2b288261c7adfd9acb065dce897caf8782aff1687efe38ba52541ef7b96.yml
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/open-transit/open-transit-49611a380a238e29ad714c2f6d66a35ada42e3931d2aad2839afd1f13b585de1.yml
 openapi_spec_hash: b03acca245aef78353d3b7a6a4a62eb2
-config_hash: c28ddf5b7754155603d9fd1c5fcaeeff
+config_hash: ff7ff57d4c7f9c3f7a4f9bae39aa00e3
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3e3e14e2..10379f97 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 1.9.1 (2026-06-17)
+
+Full Changelog: [v1.9.0...v1.9.1](https://github.com/OneBusAway/ruby-sdk/compare/v1.9.0...v1.9.1)
+
+### Bug Fixes
+
+* **client:** send content-type header for requests with an omitted optional body ([cda4fbd](https://github.com/OneBusAway/ruby-sdk/commit/cda4fbdc4803761c42392d3d270645943de81560))
+
 ## 1.9.0 (2026-06-01)
 
 Full Changelog: [v1.8.0...v1.9.0](https://github.com/OneBusAway/ruby-sdk/compare/v1.8.0...v1.9.0)
diff --git a/Gemfile.lock b/Gemfile.lock
index ecb70b6e..38cf25b3 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -11,7 +11,7 @@ GIT
 PATH
   remote: .
   specs:
-    onebusaway-sdk (1.9.0)
+    onebusaway-sdk (1.9.1)
       cgi
       connection_pool
 
diff --git a/README.md b/README.md
index 8b71e240..58f6f4da 100644
--- a/README.md
+++ b/README.md
@@ -17,7 +17,7 @@ To use this gem, install via Bundler by adding the following to your application
 <!-- x-release-please-start-version -->
 
 ```ruby
-gem "onebusaway-sdk", "~> 1.9.0"
+gem "onebusaway-sdk", "~> 1.9.1"
 ```
 
 <!-- x-release-please-end -->
diff --git a/lib/onebusaway_sdk/internal/transport/base_client.rb b/lib/onebusaway_sdk/internal/transport/base_client.rb
index 8b15a7b5..30d885e7 100644
--- a/lib/onebusaway_sdk/internal/transport/base_client.rb
+++ b/lib/onebusaway_sdk/internal/transport/base_client.rb
@@ -309,7 +309,10 @@ def initialize(
               OnebusawaySDK::Internal::Util.deep_merge(*[req[:body], opts[:extra_body]].compact)
             end
 
-          headers.delete("content-type") if body.nil?
+          # Generated methods always pass `req[:body]` for operations that define a
+          # request body, so only elide the content-type header when the operation
+          # has no body at all, not when an optional body param was omitted.
+          headers.delete("content-type") if body.nil? && !req.key?(:body)
 
           url = OnebusawaySDK::Internal::Util.join_parsed_uri(
             @base_url_components,
diff --git a/lib/onebusaway_sdk/version.rb b/lib/onebusaway_sdk/version.rb
index db9b34e5..e327c9ca 100644
--- a/lib/onebusaway_sdk/version.rb
+++ b/lib/onebusaway_sdk/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module OnebusawaySDK
-  VERSION = "1.9.0"
+  VERSION = "1.9.1"
 end