SE surface deletion crash

diff --git a/control.lua b/control.lua
index a479120..cfb7573 100755
--- a/control.lua
+++ b/control.lua
@@ -130,7 +130,7 @@ local function delete_random_decal()

     local d = secondary_range

-    if decal.decorative.valid and surface.valid then
+    if decal.decorative.valid and surface ~= nil and surface.valid then
         local anchors = surface.find_entities_filtered{name = biter_decal_anchors, position = decal.position, radius = primary_range}

         if #anchors==0 then
@@ -172,17 +172,19 @@ script.on_event(defines.events.on_chunk_deleted, function(event)
     local hitlist = {}

     for i,table_entry in ipairs(storage.biter_decal_table) do
-        for _,chunk in pairs(event.positions) do
-            local surface = table_entry.surface
-            local x = table_entry.position.x
-            local y = table_entry.position.y
-            local xmin = chunk.x*32
-            local xmax = xmin+32
-            local ymin = chunk.y*32
-            local ymax = ymin+32
-
-            if (surface == event.surface_index) and (x>=xmin) and (x<xmax) and (y>=ymin) and (y<ymax) then
-                hitlist[#hitlist+1] = i
+        if table_entry ~= nil then
+            for _,chunk in pairs(event.positions) do
+                local surface = table_entry.surface
+                local x = table_entry.position.x
+                local y = table_entry.position.y
+                local xmin = chunk.x*32
+                local xmax = xmin+32
+                local ymin = chunk.y*32
+                local ymax = ymin+32
+
+                if (surface == event.surface_index) and (x>=xmin) and (x<xmax) and (y>=ymin) and (y<ymax) then
+                    hitlist[#hitlist+1] = i
+                end
             end
         end
     end