diff --git a/README.md b/README.md
index 7c44fd5b..e48037e5 100644
--- a/README.md
+++ b/README.md
@@ -56,7 +56,7 @@ class UserSignup < ActiveInteraction::Base
   string :email, :name
 
   # optional
-  boolean :newsletter_subscribe, allow_nil: true
+  boolean :newsletter_subscribe, default: nil
 
   # ActiveRecord validations
   validates :email, format: EMAIL_REGEX
@@ -161,10 +161,10 @@ end
     integer :age
     boolean :is_special
     model :account
-    array :tags, allow_nil: true do
+    array :tags, default: nil do
       string
     end
-    hash :prefs, allow_nil: true do
+    hash :prefs, default: nil do
       boolean :smoking
       boolean :view
     end
diff --git a/lib/active_interaction/base.rb b/lib/active_interaction/base.rb
index 7ead968d..7d5c6190 100644
--- a/lib/active_interaction/base.rb
+++ b/lib/active_interaction/base.rb
@@ -15,7 +15,7 @@ module ActiveInteraction
   #     integer :a, :b
   #
   #     # Optional
-  #     integer :c, allow_nil: true
+  #     integer :c, default: nil
   #
   #     def execute
   #       sum = a + b
diff --git a/lib/active_interaction/caster.rb b/lib/active_interaction/caster.rb
index 0e51b842..2cf4ef0a 100644
--- a/lib/active_interaction/caster.rb
+++ b/lib/active_interaction/caster.rb
@@ -2,7 +2,6 @@ module ActiveInteraction
   # @!macro [new] attribute_method_params
   #   @param *attributes [Symbol] One or more attributes to create.
   #   @param options [Hash]
-  #   @option options [Boolean] :allow_nil Allow a `nil` value.
   #   @option options [Object] :default Value to use if `nil` is given.
 
   # @private
@@ -19,9 +18,7 @@ def self.factory(type)
     def self.prepare(filter, value)
       case value
         when NilClass
-          if filter.options[:allow_nil]
-            nil
-          elsif filter.options.has_key?(:default)
+          if filter.options.has_key?(:default)
             # REVIEW: This value isn't actually used anywhere. It is required
             #   to make the validator (Validation.validate) happy.
             filter.options[:default]
diff --git a/lib/active_interaction/casters/array_caster.rb b/lib/active_interaction/casters/array_caster.rb
index b912fcd2..12802b30 100644
--- a/lib/active_interaction/casters/array_caster.rb
+++ b/lib/active_interaction/casters/array_caster.rb
@@ -16,7 +16,7 @@ class Base
     #
     # @example An Array of Integers where some or all are nil
     #   array :ids do
-    #     integer allow_nil: true
+    #     integer default: nil
     #   end
     #
     # @method self.array(*attributes, options = {}, &block)
diff --git a/spec/support/casters.rb b/spec/support/casters.rb
index 1cc13ebd..bc0530a7 100644
--- a/spec/support/casters.rb
+++ b/spec/support/casters.rb
@@ -26,7 +26,7 @@
     end
 
     context 'optional' do
-      before { options.merge!(allow_nil: true) }
+      before { options.merge!(default: nil) }
 
       context 'with nil' do
         it 'returns nil' do
diff --git a/spec/support/interactions.rb b/spec/support/interactions.rb
index c96a5838..6dba57d3 100644
--- a/spec/support/interactions.rb
+++ b/spec/support/interactions.rb
@@ -19,10 +19,8 @@ def execute
   let(:described_class) do
     Class.new(TestInteraction) do
       send(type, :required, filter_options)
-      send(type, :optional, filter_options.merge(allow_nil: true))
+      send(type, :optional, filter_options.merge(default: nil))
       send(type, :default, filter_options.merge(default: generator.call))
-      send(type, :nil_default,
-           filter_options.merge(allow_nil: true, default: nil))
       send(type, :defaults_1, :defaults_2,
            filter_options.merge(default: generator.call))
 
@@ -31,7 +29,6 @@ def execute
           required: required,
           optional: optional,
           default: default,
-          nil_default: nil_default,
           defaults_1: defaults_1,
           defaults_2: defaults_2
         }
@@ -71,10 +68,6 @@ def execute
       expect(result[:default]).to_not be_nil
     end
 
-    it 'returns nil for :nil_default' do
-      expect(result[:nil_default]).to be_nil
-    end
-
     it 'does not return nil for :defaults_1' do
       expect(result[:defaults_1]).to_not be_nil
     end