diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index bd9a4a51..6c1f9421 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -679,7 +679,8 @@ module Sinatra
 
       # Sets an option to the given value.  If the value is a proc,
       # the proc will be called every time the option is accessed.
-      def set(option, value=self, &block)         
+      def set(option, value=self, &block)
+        raise ArgumentError if block && value != self
         value = block if block
         if value.kind_of?(Proc)
           metadef(option, &value)
diff --git a/test/settings_test.rb b/test/settings_test.rb
index 482fc614..bfb2bbd0 100644
--- a/test/settings_test.rb
+++ b/test/settings_test.rb
@@ -27,11 +27,11 @@ class SettingsTest < Test::Unit::TestCase
     assert_equal 'baz', @base.foo
   end
 
-  # TODO: should it raise an error instead?
-  it 'ignores any other value if set using a block' do
-    @base.set(:foo, 'ignore'){ 'baz' }
-    assert @base.respond_to?(:foo)
-    assert_equal 'baz', @base.foo
+  it 'raises an error with a value and a block' do
+    assert_raise ArgumentError do
+      @base.set(:fiz, 'boom!'){ 'baz' }
+    end
+    assert !@base.respond_to?(:fiz)
   end
 
   it "sets multiple settings with a Hash" do