diff --git a/gtk/theme/Adwaita/Gemfile b/gtk/theme/Adwaita/Gemfile
index 99f8e2a637..56af69e67a 100644
--- a/gtk/theme/Adwaita/Gemfile
+++ b/gtk/theme/Adwaita/Gemfile
@@ -1 +1,2 @@
+source 'https://rubygems.org'
 gem "sass", "~> 3.4.0"
diff --git a/gtk/theme/HighContrast/Gemfile b/gtk/theme/HighContrast/Gemfile
index 99f8e2a637..56af69e67a 100644
--- a/gtk/theme/HighContrast/Gemfile
+++ b/gtk/theme/HighContrast/Gemfile
@@ -1 +1,2 @@
+source 'https://rubygems.org'
 gem "sass", "~> 3.4.0"