diff --git a/rb/lib/selenium/webdriver/bidi/serialization/union.rb b/rb/lib/selenium/webdriver/bidi/serialization/union.rb
index 7eb19754f9682..cd4821105f4c8 100644
--- a/rb/lib/selenium/webdriver/bidi/serialization/union.rb
+++ b/rb/lib/selenium/webdriver/bidi/serialization/union.rb
@@ -53,14 +53,18 @@ def object_only = @object_only = true
             # An outbound scalar outside that set matches no arm, so it is a caller error.
             def scalar_values(*values) = @scalar_values = values
 
-            # A non-Hash payload is a bare scalar arm (e.g. input.Origin's "viewport") with no
-            # object to dispatch on, so it is returned unchanged — unless every arm is an object
-            # (object_only), where a non-Hash cannot match any variant and is a wire error.
+            # A non-Hash payload is a bare scalar arm (e.g. input.Origin's "viewport"), valid only
+            # as a literal the schema pins; under object_only it cannot match any variant at all.
             def from_json(json_payload)
               unless json_payload.is_a?(::Hash)
-                return json_payload unless @object_only
+                if @object_only
+                  raise Error::SerializationError,
+                        "#{name} expected an object on the wire, got #{json_payload.inspect}"
+                end
+                return json_payload if scalar_arm?(json_payload)
 
-                raise Error::SerializationError, "#{name} expected an object on the wire, got #{json_payload.inspect}"
+                raise Error::SerializationError,
+                      "#{name} received a scalar not in this Selenium's BiDi schema: #{json_payload.inspect}"
               end
 
               variant = select(json_payload)
diff --git a/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb b/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
index b3425197c6298..73eeb893aa762 100644
--- a/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
+++ b/rb/spec/unit/selenium/webdriver/bidi/serialization_spec.rb
@@ -87,10 +87,15 @@ def valid_cookie_attrs
                 .to raise_error(Error::SerializationError, /RemoteValue expected an object/)
             end
 
-            it 'passes a bare scalar through a union that has a scalar arm (input.Origin)' do
+            it 'passes a declared bare scalar through a union that has a scalar arm (input.Origin)' do
               expect(Input::Origin.from_json('viewport')).to eq('viewport')
             end
 
+            it 'raises when a bare scalar is not one of the union scalar arms' do
+              expect { Input::Origin.from_json('banana') }
+                .to raise_error(Error::SerializationError, /Origin received a scalar not in this Selenium/)
+            end
+
             it 'keeps a map string key while typing its object value (object-only value union)' do
               parsed = Script::ObjectRemoteValue.from_json(
                 'type' => 'object', 'value' => [['k', {'type' => 'number', 'value' => 2}]]