diff --git a/marimo/_output/utils.py b/marimo/_output/utils.py
index 4e9e92307c6..fa2a520e8a8 100644
--- a/marimo/_output/utils.py
+++ b/marimo/_output/utils.py
@@ -8,6 +8,7 @@
 
 
 def build_data_url(mimetype: KnownMimeType, data: bytes) -> str:
+    # `data` must be base64 encoded
     str_repr = data.decode("utf-8").replace("\n", "")
     return f"data:{mimetype};base64,{str_repr}"
 
diff --git a/marimo/_server/export/exporter.py b/marimo/_server/export/exporter.py
index df6a6fd4ba9..fe6d9f738d1 100644
--- a/marimo/_server/export/exporter.py
+++ b/marimo/_server/export/exporter.py
@@ -1,6 +1,7 @@
 # Copyright 2024 Marimo. All rights reserved.
 from __future__ import annotations
 
+import base64
 import io
 import mimetypes
 import os
@@ -61,7 +62,8 @@ def export_as_html(
                 None,
             )
             files[filename_and_length] = build_data_url(
-                cast(KnownMimeType, mime_type), buffer_contents
+                cast(KnownMimeType, mime_type),
+                base64.b64encode(buffer_contents),
             )
 
         config = _deep_copy(DEFAULT_CONFIG)