Raise error when fails to find a given wrapper.

lib/simple_form.rb

@@ -117,7 +117,15 @@ module SimpleForm
 
   # Retrieves a given wrapper
   def self.wrapper(name)
-    @@wrappers[name]
+    if wrapper = @@wrappers[name]
+      wrapper
+    else
+      raise WrapperNotFound, "Couldn't find wrapper with name #{name}"
+    end
+  end
+
+  # Raised when fails to find a given wrapper name
+  class WrapperNotFound < StandardError
   end
 
   # Define a new wrapper using SimpleForm::Wrappers::Builder
@@ -166,4 +174,4 @@ module SimpleForm
         "Updating to the new API is easy and fast. Check for more info here: https://github.com/plataformatec/simple_form/wiki/Upgrading-to-Simple-Form-2.0"
     end
   end
-end
+end

lib/simple_form/form_builder.rb

@@ -93,7 +93,7 @@ module SimpleForm
     def input(attribute_name, options={}, &block)
       chosen =
         if name = options[:wrapper]
-          name.is_a?(Symbol) ? SimpleForm.wrapper(name) : name
+          name.respond_to?(:render) ? name : SimpleForm.wrapper(name)
         else
           wrapper
         end

test/form_builder/wrapper_test.rb

@@ -112,4 +112,10 @@ class WrapperTest < ActionView::TestCase
     assert_select "section.custom_wrapper div.another_wrapper label"
     assert_select "section.custom_wrapper div.another_wrapper input.string"
   end
+
+  test 'raise error when wrapper not found' do
+    assert_raise SimpleForm::WrapperNotFound do
+      with_form_for @user, :name, :wrapper => :not_found
+    end
+  end
 end