Dokku push error

After several try to push an app to a dokku instance . i keep receiving this error.I've already search both github and stackoverflow for similar issues but none of them resolve it.I have check my css folders and files for any issue .Unable to pinpoint where the error coming from. If anybody have an idea to resolve it . please share.

 NoMethodError: undefined method `[]' for nil:NilClass                                                                                                                                 
       (in /tmp/build/app/assets/stylesheets/application.css)                                                                                                                                
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sprockets-2.12.4/lib/sprockets/sass_functions.rb:63:in `sprockets_context'                                                                   
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sprockets-2.12.4/lib/sprockets/sass_functions.rb:14:in `image_path'                                                                          
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/funcall.rb:113:in `_perform'                                                                                     
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/node.rb:40:in `perform'                                                                                          
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `block in _perform'                                                                                
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `map'                                                                                              
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `_perform'                                                                                         
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/script/node.rb:40:in `perform'                                                                                          
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:298:in `visit_prop'                                                                            
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:37:in `visit'                                                                                     
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:100:in `visit'                                                                                 
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'                                                                   
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:53:in `map'                                                                                       
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:53:in `visit_children'                                                                            
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:109:in `block in visit_children'                                                               
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:121:in `with_environment'                                                                      
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:108:in `visit_children'                                                                        
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:37:in `block in visit'                                                                            
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:320:in `visit_rule'                                                                            
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:37:in `visit'                                                                                     
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:100:in `visit'                                                                                 
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:53:in `block in visit_children'                                                                   
       /tmp/build/vendor/bundle/ruby/2.0.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:53:in `map'