diff --git a/app/models/theme_field.rb b/app/models/theme_field.rb
index da1ab9d2a29..92d7fd2d172 100644
--- a/app/models/theme_field.rb
+++ b/app/models/theme_field.rb
@@ -44,10 +44,12 @@ class ThemeField < ActiveRecord::Base
   def transpile(es6_source, version)
     template = Tilt::ES6ModuleTranspilerTemplate.new {}
     wrapped = <<PLUGIN_API_JS
+if ('Discourse' in window) {
 Discourse._registerPluginCode('#{version}', api => {
   #{settings(es6_source)}
   #{es6_source}
 });
+}
 PLUGIN_API_JS
 
     template.babel_transpile(wrapped)
@@ -73,7 +75,9 @@ PLUGIN_API_JS
         node.replace <<COMPILED
           <script>
             (function() {
+              if ('Discourse' in window) {
               Discourse.RAW_TEMPLATES[#{name.sub(/\.raw$/, '').inspect}] = #{template};
+              }
             })();
           </script>
 COMPILED
@@ -82,7 +86,9 @@ COMPILED
         node.replace <<COMPILED
           <script>
             (function() {
+              if ('Em' in window) {
               Ember.TEMPLATES[#{name.inspect}] = #{template};
+              }
             })();
           </script>
 COMPILED
diff --git a/spec/models/theme_spec.rb b/spec/models/theme_spec.rb
index 5802de76120..d12e0fd10a0 100644
--- a/spec/models/theme_spec.rb
+++ b/spec/models/theme_spec.rb
@@ -233,10 +233,12 @@ HTML
       theme.save!
 
       transpiled = <<~HTML
-      <script>Discourse._registerPluginCode('1.0', function (api) {
-        var settings = { "name": "bob" };
-        alert(settings.name);var a = function a() {};
-      });</script>
+      <script>if ('Discourse' in window) {
+        Discourse._registerPluginCode('1.0', function (api) {
+          var settings = { "name": "bob" };
+          alert(settings.name);var a = function a() {};
+        });
+      }</script>
       HTML
 
       expect(Theme.lookup_field(theme.id, :desktop, :after_header)).to eq(transpiled.strip)
@@ -245,10 +247,12 @@ HTML
       setting.value = 'bill'
 
       transpiled = <<~HTML
-      <script>Discourse._registerPluginCode('1.0', function (api) {
-        var settings = { "name": "bill" };
-        alert(settings.name);var a = function a() {};
-      });</script>
+      <script>if ('Discourse' in window) {
+        Discourse._registerPluginCode('1.0', function (api) {
+          var settings = { "name": "bill" };
+          alert(settings.name);var a = function a() {};
+        });
+      }</script>
       HTML
       expect(Theme.lookup_field(theme.id, :desktop, :after_header)).to eq(transpiled.strip)