diff --git a/render.js b/render.js
index f9504c9..3d3ba34 100644
--- a/render.js
+++ b/render.js
@@ -245,7 +245,8 @@
   },
 
   sum: function(l) {
-    var acc = 0;
+    var acc;
+    if (l.length) acc = 0;
     for (var i in l) {
       acc += parseFloat(l[i]);
     }
@@ -295,7 +296,9 @@
       var outcoli = key.length + parseInt(valuei);
       var cell = row[incoli];
       if (!orow[outcoli]) orow[outcoli] = [];
-      orow[outcoli].push(cell);
+      if (cell != null) {
+	orow[outcoli].push(cell);
+      }
     }
   };