diff --git a/composer/lib/dependabot/composer/update_checker/version_resolver.rb b/composer/lib/dependabot/composer/update_checker/version_resolver.rb
index ddfdda34fd..cc6161a55f 100644
--- a/composer/lib/dependabot/composer/update_checker/version_resolver.rb
+++ b/composer/lib/dependabot/composer/update_checker/version_resolver.rb
@@ -301,7 +301,7 @@ def handle_composer_errors(error)
             # If there *is* a lockfile we can't confidently distinguish between
             # cases where we can't install and cases where we can't update. For
             # now, we therefore just ignore the dependency.
-            nil
+            raise Dependabot::DependencyFileNotResolvable, sanitized_message
           elsif error.message.include?("URL required authentication") ||
                 error.message.include?("403 Forbidden")
             source = error.message.match(%r{https?://(?<source>[^/]+)/}).named_captures.fetch("source")