diff --git a/app/helpers/boxes_helper.rb b/app/helpers/boxes_helper.rb
index a218d74..3d0b653 100644
--- a/app/helpers/boxes_helper.rb
+++ b/app/helpers/boxes_helper.rb
@@ -232,7 +232,7 @@ module BoxesHelper
   end
 
   def block_css_class_name(block)
-    block.class.name.underscore.gsub('_', '-')
+    block.class.name.underscore.gsub('_', '-').gsub('/','_')
   end
   def block_css_classes(block)
     classes = block_css_class_name(block)
diff --git a/plugins/container_block/public/style.css b/plugins/container_block/public/style.css
index a4ba851..06f8b4f 100644
--- a/plugins/container_block/public/style.css
+++ b/plugins/container_block/public/style.css
@@ -1,46 +1,46 @@
-#content .boxes .container-block-plugin/container-block .container_block_child, .container-block-plugin/container-block .block-outer {
+#content .boxes .container-block-plugin_container-block .container_block_child, .container-block-plugin_container-block .block-outer {
   display: inline-block;
   vertical-align: top;
   margin-left: -2px;
   margin-right: -2px;
 }
 
-.container-block-plugin/container-block .block-target {
+.container-block-plugin_container-block .block-target {
   clear: both;
 }
 
-.container-block-plugin/container-block .block-target[id^='end-of-box-'] {
+.container-block-plugin_container-block .block-target[id^='end-of-box-'] {
   display: none;
 }
 
-#content .boxes .container-block-plugin/container-block .block .icon-down, #content .boxes .container-block-plugin/container-block .block .icon-down-disabled {
+#content .boxes .container-block-plugin_container-block .block .icon-down, #content .boxes .container-block-plugin_container-block .block .icon-down-disabled {
   background-image: url(/designs/icons/default/Tango/16x16/actions/go-next.png);
 }
 
-#content .boxes .container-block-plugin/container-block .block .icon-up, #content .boxes .container-block-plugin/container-block .block .icon-up-disabled {
+#content .boxes .container-block-plugin_container-block .block .icon-up, #content .boxes .container-block-plugin_container-block .block .icon-up-disabled {
   background-image: url(/designs/icons/default/Tango/16x16/actions/go-previous.png);
 }
 
-#content .boxes .container-block-plugin/container-block .block {
+#content .boxes .container-block-plugin_container-block .block {
   outline-offset: -2px;
 }
 
-#content .boxes .container-block-plugin/container-block .block .ui-resizable-handle {
+#content .boxes .container-block-plugin_container-block .block .ui-resizable-handle {
   width: 10px;
   height: 28px;
   z-index: 0;
 }
 
-#content .boxes .container-block-plugin/container-block .block .ui-resizable-e {
+#content .boxes .container-block-plugin_container-block .block .ui-resizable-e {
   right: -2px;
   background-image: url(/plugins/container_block/images/handle_e.png);
 }
 
-#content .boxes .container-block-plugin/container-block .block .ui-resizable-w {
+#content .boxes .container-block-plugin_container-block .block .ui-resizable-w {
   left: -2px;
   background-image: url(/plugins/container_block/images/handle_w.png);
 }
 
-.container-block-plugin/container-block .button-bar .icon-resize {
+.container-block-plugin_container-block .button-bar .icon-resize {
   background-image: url(/designs/icons/default/Tango/16x16/actions/view-fullscreen.png);
 }
diff --git a/test/unit/boxes_helper_test.rb b/test/unit/boxes_helper_test.rb
index bf3e5b2..f2be3ea 100644
--- a/test/unit/boxes_helper_test.rb
+++ b/test/unit/boxes_helper_test.rb
@@ -97,8 +97,10 @@ class BoxesHelperTest < ActiveSupport::TestCase
   end
 
   should 'calculate CSS class names correctly' do
+    class NamespacedBlock < Block; end
     assert_equal 'slideshow-block', block_css_class_name(SlideshowBlock.new)
     assert_equal 'main-block', block_css_class_name(MainBlock.new)
+    assert_equal 'boxes-helper-test_namespaced-block', block_css_class_name(NamespacedBlock.new)
   end
 
   should 'add invisible CSS class name for invisible blocks' do
--
libgit2 0.21.2