diff --git a/lib/autoproj/local_package_set.rb b/lib/autoproj/local_package_set.rb
index b0b0c7e5..f8abd599 100644
--- a/lib/autoproj/local_package_set.rb
+++ b/lib/autoproj/local_package_set.rb
@@ -81,6 +81,9 @@ def raw_description_file
             manifest_data = Autoproj.in_file(manifest_path, Autoproj::YAML_LOAD_ERROR) do
                 YAML.load(File.read(manifest_path)) || Hash.new
             end
+            if !manifest_data["package_sets"].is_a?(Array)
+                raise SyntaxError.new "The package_sets field in your manifest file is not an array, check your YAML syntax"
+            end
             description["imports"] = description["imports"]
                                      .concat(manifest_data["package_sets"] || Array.new)
             description["name"] = name