diff --git a/grunt/tasks/build.js b/grunt/tasks/build.js
index 2a9543412..9b738f506 100644
--- a/grunt/tasks/build.js
+++ b/grunt/tasks/build.js
@@ -5,6 +5,7 @@ module.exports = function(grunt) {
   grunt.registerTask('build', 'Creates a production-ready build of the course', [
     '_log-vars',
     'check-json',
+    'check-plugins',
     'clean:output',
     'build-config',
     'tracking-insert',
diff --git a/grunt/tasks/check-json.js b/grunt/tasks/check-json.js
index 383b25507..b4d09bd6b 100644
--- a/grunt/tasks/check-json.js
+++ b/grunt/tasks/check-json.js
@@ -3,6 +3,7 @@ module.exports = function(grunt) {
   grunt.registerTask('check-json', 'Validates the course json, checks for duplicate IDs, and that each element has a parent', function() {
     // validates JSON files
     grunt.task.run('jsonlint');
+    grunt.task.run('check-plugins');
     const data = Helpers.getFramework().getData();
     data.checkIds();
   });
diff --git a/grunt/tasks/check-plugins.js b/grunt/tasks/check-plugins.js
new file mode 100644
index 000000000..745c32d34
--- /dev/null
+++ b/grunt/tasks/check-plugins.js
@@ -0,0 +1,27 @@
+module.exports = function(grunt) {
+  const Helpers = require('../helpers')(grunt);
+  grunt.registerTask('check-plugins', 'Checks that only one theme is active in the build', function() {
+    // A specific theme was selected via CLI flag (e.g. --theme=X),
+    // so only that one will be compiled regardless of what's installed
+    if (grunt.config('theme') !== '**') return;
+
+    const includes = grunt.config('includes');
+    const excludes = [
+      ...(grunt.config('excludes') || []),
+      ...(grunt.config('type') === 'production' ? (grunt.config('productionExcludes') || []) : [])
+    ];
+
+    const installed = Helpers.getInstalledPluginsByType('theme');
+    const active = includes
+      ? installed.filter(name => includes.includes(name))
+      : installed.filter(name => !excludes.includes(name));
+
+    if (active.length <= 1) return;
+
+    grunt.fail.fatal(
+      `More than one theme is active in this build: ${active.join(', ')}.\n` +
+      `Add the extras to build.excludes in config.json, e.g.:\n` +
+      `  "build": { "excludes": ["${active.slice(1).join('", "')}"] }`
+    );
+  });
+};
diff --git a/grunt/tasks/dev.js b/grunt/tasks/dev.js
index 7c7335add..0797144fe 100644
--- a/grunt/tasks/dev.js
+++ b/grunt/tasks/dev.js
@@ -5,6 +5,7 @@ module.exports = function(grunt) {
   grunt.registerTask('dev', 'Creates a developer-friendly build of the course', [
     '_log-vars',
     'check-json',
+    'check-plugins',
     'build-config',
     'tracking-insert',
     'copy',