diff --git a/lib/propshaft/compiler/css_asset_urls.rb b/lib/propshaft/compiler/css_asset_urls.rb
index de8a950..5e9a737 100644
--- a/lib/propshaft/compiler/css_asset_urls.rb
+++ b/lib/propshaft/compiler/css_asset_urls.rb
@@ -13,7 +13,7 @@ def referenced_by(asset, references: Set.new)
     asset.content.scan(ASSET_URL_PATTERN).each do |referenced_asset_url, _|
       referenced_asset = load_path.find(resolve_path(asset.logical_path.dirname, referenced_asset_url))
 
-      unless references.include?(referenced_asset)
+      if referenced_asset && references.exclude?(referenced_asset)
         references << referenced_asset
         references.merge referenced_by(referenced_asset, references: references)
       end
diff --git a/test/fixtures/assets/first_path/dependent/b.css b/test/fixtures/assets/first_path/dependent/b.css
index 5a89d0f..084de15 100644
--- a/test/fixtures/assets/first_path/dependent/b.css
+++ b/test/fixtures/assets/first_path/dependent/b.css
@@ -1 +1,2 @@
-@import url('c.css')
\ No newline at end of file
+@import url('c.css')
+@import url('missing.css')