Upgrading to Rails 4 Sprockets::ConversionError

I am in the process of upgrading my app to rails 4. On stylesheet_link_tag or image_tag I get the error:

Sprockets::ConversionError at / could not convert “text/html” to nil

In the console:


after some time I get the same error.

I have emptied the sprockets cache and use sprockets 3.5.2.

Any help much appreciated,


Sorted. I found some unexpected files in stylesheets/images.

Sorry to post,