diff --git a/IPython/frontend/html/notebook/static/less/tooltip.less b/IPython/frontend/html/notebook/static/less/tooltip.less
index 297d8a9..cc78517 100644
--- a/IPython/frontend/html/notebook/static/less/tooltip.less
+++ b/IPython/frontend/html/notebook/static/less/tooltip.less
@@ -117,6 +117,8 @@
     };
     position: absolute;
 
+	z-index: 2;
+
 }
 
 .pretooltiparrow {