diff --git a/sphinx/templates/modindex.html b/sphinx/templates/modindex.html
index 692013578..d6b505da7 100644
--- a/sphinx/templates/modindex.html
+++ b/sphinx/templates/modindex.html
@@ -46,7 +46,7 @@
    <tr{% if indent %} class="cg-{{ cgroup }}"{% endif %}>
      <td>{% if collapse -%}
        <img src="{{ pathto('_static/minus.png', 1) }}" id="toggle-{{ cgroup }}"
-            class="toggler" style="display: none" />
+            class="toggler" style="display: none" alt="-" />
          {%- endif %}</td>
      <td>{% if indent %}&nbsp;&nbsp;&nbsp;{% endif %}
      {% if fname %}<a href="{{ fname }}">{% endif -%}