diff --git a/lms/static/sass/application.scss.mako b/lms/static/sass/application.scss.mako
index 434475c6ac..c310347b6f 100644
--- a/lms/static/sass/application.scss.mako
+++ b/lms/static/sass/application.scss.mako
@@ -45,7 +45,7 @@
 ## called themes/<theme-name>/, with its base Sass file in
 ## themes/<theme-name>/static/sass/_<theme-name>.scss. That one entry
 ## point can be used to @import in as many other things as needed.
-% if not env['THEME_NAME'] is None:
+% if env.get('THEME_NAME') is not None:
   // import theme's Sass overrides
-  @import '${env['THEME_NAME']}'
+  @import '${env.get('THEME_NAME')}'
 % endif
diff --git a/rakefiles/assets.rake b/rakefiles/assets.rake
index b7edaa1b02..c0757b712b 100644
--- a/rakefiles/assets.rake
+++ b/rakefiles/assets.rake
@@ -19,9 +19,11 @@ def preprocess_with_mako(filename)
     # strip off the .mako extension
     output_filename = filename.chomp(File.extname(filename))
 
-    # just pipe from stdout into the new file
+    # just pipe from stdout into the new file, exiting on failure
     File.open(output_filename, 'w') do |file|
       file.write(`python -c '#{mako}'`)
+      exit_code = $?.to_i
+      abort "#{mako} failed with #{exit_code}" if exit_code.to_i != 0
     end
 end