diff --git a/lib/definitions/evaluator.rb b/lib/definitions/evaluator.rb
index 9be7fca..3556c18 100644
--- a/lib/definitions/evaluator.rb
+++ b/lib/definitions/evaluator.rb
@@ -40,26 +40,35 @@ def instance_evaluate(proxy:)
       eval(proxy.value, binding, proxy.file_path, proxy.start_line) # rubocop:disable Security/Eval
     end
 
+    def class_evaluate(proxy:, class_binding:)
+      # Not a security risk because the code comes from a trusted source; the file that included lowtype.
+      eval(proxy.value, class_binding, proxy.file_path, proxy.start_line) # rubocop:disable Security/Eval
+    end
+
     class << self
-      def evaluate(method_proxies:)
+      def evaluate(method_proxies:, class_binding: nil)
         require_relative '../syntax/union_types' if LowType.config.union_type_expressions
 
         method_proxies.each_value do |method_proxy|
-          evaluate_param_proxy_expressions(method_proxy:)
+          evaluate_param_proxy_expressions(method_proxy:, class_binding:)
           evaluate_return_proxy_expression(return_proxy: method_proxy.return_proxy) if method_proxy.return_proxy
         end
       end
 
-      def evaluate_param_proxy_expressions(method_proxy:)
+      def evaluate_param_proxy_expressions(method_proxy:, class_binding: nil)
         begin # rubocop:disable Style/RedundantBegin
           method_proxy.tagged_params(:value).each do |param_proxy|
-            # TODO: Evaluate in the binding of the class that included LowType if not a type managed by LowType.
-            expression = new.instance_evaluate(proxy: param_proxy)
+            expression = begin
+              new.instance_evaluate(proxy: param_proxy)
+            rescue NameError
+              raise unless class_binding
+              new.class_evaluate(proxy: param_proxy, class_binding:)
+            end
             param_proxy.expression = cast_type_expression(expression:, method_proxy:)
           end
-        rescue NameError
+        rescue NameError => e
           mp = method_proxy
-          raise NameError, "Unknown type '#{mp.value}' for #{mp.scope} at #{mp.file_path}:#{mp.start_line}"
+          raise NameError, "Unknown type '#{e.name}' for #{mp.scope} at #{mp.file_path}:#{mp.start_line}"
         end
       end
 
diff --git a/lib/low_type.rb b/lib/low_type.rb
index 50fbd4c..304d430 100644
--- a/lib/low_type.rb
+++ b/lib/low_type.rb
@@ -50,7 +50,7 @@ def self.included(klass)
 
       class_proxy.class_binding = trace.binding
 
-      Low::Evaluator.evaluate(method_proxies: class_proxy.keyed_methods)
+      Low::Evaluator.evaluate(method_proxies: class_proxy.keyed_methods, class_binding: class_proxy.class_binding)
 
       klass.prepend Low::Redefiner.redefine(method_proxies: class_proxy.instance_methods, class_proxy:)
       klass.singleton_class.prepend Low::Redefiner.redefine(method_proxies: class_proxy.class_methods, class_proxy:)
diff --git a/spec/features/custom_class_spec.rb b/spec/features/custom_class_spec.rb
new file mode 100644
index 0000000..8559fa9
--- /dev/null
+++ b/spec/features/custom_class_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require_relative '../fixtures/custom_class'
+
+RSpec.describe Order do
+  subject { described_class.new }
+
+  describe '#process' do
+    it 'accepts a user-defined class as a type without raising NameError' do
+      expect { subject.process(method: PaymentMethod.new) }.not_to raise_error
+    end
+
+    it 'returns the passed value' do
+      payment = PaymentMethod.new
+      expect(subject.process(method: payment)).to eq(payment)
+    end
+  end
+end
diff --git a/spec/fixtures/custom_class.rb b/spec/fixtures/custom_class.rb
new file mode 100644
index 0000000..520e8c0
--- /dev/null
+++ b/spec/fixtures/custom_class.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require_relative '../../lib/low_type'
+
+class PaymentMethod; end
+
+class Order
+  include LowType
+
+  def process(method: PaymentMethod)
+    method
+  end
+end