diff --git a/app/models/hiera_data/hierarchy.rb b/app/models/hiera_data/hierarchy.rb
index d532dc20..f67d3c48 100644
--- a/app/models/hiera_data/hierarchy.rb
+++ b/app/models/hiera_data/hierarchy.rb
@@ -99,7 +99,7 @@ def determine_backend
       value = raw_hash[lookup_function]
       backends = BACKENDS
       custom_mappings = Rails.configuration.hdm[:custom_lookup_function_mapping]
-      backends = backends.deep_merge({ "lookup_key" => custom_mappings }) if custom_mappings.present?
+      backends = backends.deep_merge({ "lookup_key" => custom_mappings.stringify_keys }) if custom_mappings.present?
       backends.fetch(key).fetch(value).to_sym
     rescue KeyError
       raise Hdm::Error, "unknown backend #{value}"
diff --git a/test/models/hiera_data/hierarchy_test.rb b/test/models/hiera_data/hierarchy_test.rb
index 84ce0cd6..253c063f 100644
--- a/test/models/hiera_data/hierarchy_test.rb
+++ b/test/models/hiera_data/hierarchy_test.rb
@@ -234,7 +234,7 @@ class HierarchyForCustomBackend < ActiveSupport::TestCase
 
       test "custom lookup function mapped to eyaml" do
         Rails.configuration.hdm[:custom_lookup_function_mapping] = {
-          "custom_eyaml_function" => "eyaml"
+          custom_eyaml_function: "eyaml"
         }
         hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
         assert_equal :eyaml, hierarchy.backend