diff --git a/lib/librarian/manifest_set.rb b/lib/librarian/manifest_set.rb
index 6e966f2e..3ee5cf91 100644
--- a/lib/librarian/manifest_set.rb
+++ b/lib/librarian/manifest_set.rb
@@ -137,6 +137,7 @@ def dependencies_of(names)
         next if deps.include?(name)
 
         deps << name
+        raise(Error, "Unable to find module #{name}") if index[name].nil?
         names.concat index[name].dependencies.map(&:name)
       end
       deps.to_a