diff --git a/src/Forms/HTMLEditor/TinyMCECombinedGenerator.php b/src/Forms/HTMLEditor/TinyMCECombinedGenerator.php
index 90fb5c116..72c384cd8 100644
--- a/src/Forms/HTMLEditor/TinyMCECombinedGenerator.php
+++ b/src/Forms/HTMLEditor/TinyMCECombinedGenerator.php
@@ -78,8 +78,7 @@ class TinyMCECombinedGenerator implements TinyMCEScriptGenerator, Flushable
     {
         $tinymceDir = $config->getTinyMCEPath();
 
-        // Core JS file
-        $files = [ $tinymceDir . '/tinymce' ];
+        $files = [ ];
 
         // Add core languages
         $language = $config->getOption('language');
@@ -137,18 +136,43 @@ class TinyMCECombinedGenerator implements TinyMCEScriptGenerator, Flushable
             return null;
         }, $files));
 
-        // Set base URL for where tinymce is loaded from
-        $buffer = "var tinyMCEPreInit={base:'" . Convert::raw2js($tinymceDir) . "',suffix:'.min'};\n";
+        $libContent = $this->getFileContents(Director::baseFolder() . '/' . $tinymceDir . '/tinymce.min.js');
+
+        // Register vars for config
+        $baseDirJS = Convert::raw2js(Director::absoluteBaseURL());
+        $buffer = [];
+        $buffer[] = <<<SCRIPT
+(function() {
+  var baseTag = window.document.getElementsByTagName('base');
+  var baseURL = baseTag.length ? baseTag[0].baseURI : '$baseDirJS';
+SCRIPT;
+        $buffer[] = <<<SCRIPT
+(function() {
+  // Avoid double-registration
+  if (window.tinymce) {
+    return;
+  }
+
+  var tinyMCEPreInit = {
+    base: baseURL,
+    suffix: '.min',
+  };
+  $libContent
+})();
+SCRIPT;
 
         // Load all tinymce script files into buffer
         foreach ($files as $file) {
-            $buffer .= $this->getFileContents(Director::baseFolder() . '/' . $file) . "\n";
+            $buffer[] = $this->getFileContents(Director::baseFolder() . '/' . $file);
         }
 
+        $filesList = Convert::raw2js(implode(',', $files));
         // Mark all themes, plugins and languages as done
-        $buffer .= 'tinymce.each("' . Convert::raw2js(implode(',', $files)) . '".split(","),function(f){tinymce.ScriptLoader.markDone(f);});';
+        $buffer[] = "window.tinymce.each('$filesList'.split(','),".
+            "function(f){tinymce.ScriptLoader.markDone(baseURL+f);});";
 
-        return $buffer . "\n";
+        $buffer[] = '})();';
+        return implode("\n", $buffer) . "\n";
     }
 
 
diff --git a/tests/php/Forms/HTMLEditor/HTMLEditorFieldTest.php b/tests/php/Forms/HTMLEditor/HTMLEditorFieldTest.php
index ac3a960b5..7030601bc 100644
--- a/tests/php/Forms/HTMLEditor/HTMLEditorFieldTest.php
+++ b/tests/php/Forms/HTMLEditor/HTMLEditorFieldTest.php
@@ -9,6 +9,8 @@ use SilverStripe\Assets\Folder;
 use SilverStripe\Assets\Image;
 use SilverStripe\Assets\Tests\Storage\AssetStoreTest\TestAssetStore;
 use SilverStripe\Core\Config\Config;
+use SilverStripe\Core\Manifest\ModuleLoader;
+use SilverStripe\Core\Manifest\ModuleManifest;
 use SilverStripe\Dev\CSSContentParser;
 use SilverStripe\Dev\FunctionalTest;
 use SilverStripe\Forms\HTMLEditor\HTMLEditorField;
@@ -66,7 +68,13 @@ class HTMLEditorFieldTest extends FunctionalTest
     public function testCasting()
     {
         // Shim TinyMCE so silverstripe/admin doesn't have to be installed
-        TinyMCEConfig::config()->set('base_dir', 'test');
+        $framework = ModuleLoader::getModule('silverstripe/framework');
+        TinyMCEConfig::config()->set(
+            'base_dir',
+            $framework->getRelativeResourcePath(
+                'tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest/tinymce'
+            )
+        );
         HtmlEditorField::config()->set('use_gzip', false);
 
         // Test special characters
diff --git a/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest.php b/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest.php
index ff47b8514..55532935a 100644
--- a/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest.php
+++ b/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest.php
@@ -4,6 +4,7 @@ namespace SilverStripe\Forms\Tests\HTMLEditor;
 
 use SilverStripe\Control\Director;
 use SilverStripe\Core\Injector\Injector;
+use SilverStripe\Core\Manifest\ModuleLoader;
 use SilverStripe\Dev\SapphireTest;
 use SilverStripe\Forms\HTMLEditor\HTMLEditorConfig;
 use SilverStripe\Forms\HTMLEditor\TinyMCECombinedGenerator;
@@ -59,7 +60,10 @@ class TinyMCECombinedGeneratorTest extends SapphireTest
             "Filename for config: " . json_encode($c->getAttributes()) . " should match expected value"
         );
         $content = $generator->generateContent($c);
-        $this->assertStringStartsWith("var tinyMCEPreInit={base:'tinymce',suffix:'.min'};\n", $content);
+        $this->assertContains(
+            "var baseURL = baseTag.length ? baseTag[0].baseURI : 'http://www.mysite.com/basedir/';\n",
+            $content
+        );
         // Main script file
         $this->assertContains("/* tinymce.js */\n", $content);
         // Locale file
@@ -81,10 +85,9 @@ class TinyMCECombinedGeneratorTest extends SapphireTest
         $this->assertContains("/* testtheme/langs/en.js */\n", $content);
 
         // Register done scripts
-        $this->assertStringEndsWith(
+        $this->assertContains(
             <<<EOS
-tinymce.each("tinymce/tinymce.js,tinymce/langs/en.js,mycode/plugin1.js,tinymce/plugins/plugin4/plugin.min.js,tinymce/plugins/plugin4/langs/en.js,tinymce/plugins/plugin5/plugin.js,mycode/plugin6.js,tinymce/themes/testtheme/theme.js,tinymce/themes/testtheme/langs/en.js".split(","),function(f){tinymce.ScriptLoader.markDone(f);});
-
+tinymce.each('tinymce/langs/en.js,mycode/plugin1.js,tinymce/plugins/plugin4/plugin.min.js,tinymce/plugins/plugin4/langs/en.js,tinymce/plugins/plugin5/plugin.js,mycode/plugin6.js,tinymce/themes/testtheme/theme.js,tinymce/themes/testtheme/langs/en.js'.split(','),function(f){tinymce.ScriptLoader.markDone(baseURL+f);});
 EOS
             ,
             $content
diff --git a/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest/tinymce/tinymce.js b/tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest/tinymce/tinymce.min.js
similarity index 100%
rename from tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest/tinymce/tinymce.js
rename to tests/php/Forms/HTMLEditor/TinyMCECombinedGeneratorTest/tinymce/tinymce.min.js