diff --git a/grade/report/grader/templates/collapse/icon.mustache b/grade/report/grader/templates/collapse/icon.mustache
index c465330d657..46e444f21a5 100644
--- a/grade/report/grader/templates/collapse/icon.mustache
+++ b/grade/report/grader/templates/collapse/icon.mustache
@@ -23,7 +23,7 @@
         "name": "Meaning of life"
     }
 }}
-<button class="btn btn-link btn-icon icon-size-3" data-hider="expand" data-col="{{field}}">
+<button type="button" class="btn btn-link btn-icon icon-size-3" data-hider="expand" data-col="{{field}}">
     <i class="icon fa fa-plus m-0" title="Reopen column" aria-hidden="true"></i>
     <span class="sr-only">{{#str}}reopencolumn, gradereport_grader, {{name}}{{/str}}</span>
 </button>