diff --git a/compacting-storage.lua b/compacting-storage.lua
index 1b5d36d..7943108 100644
--- a/compacting-storage.lua
+++ b/compacting-storage.lua
@@ -129,8 +129,14 @@ function should_fill (type, counts)
                 amount_needed = diff
             end
         else
-            fill_src = form
-            amount_spare = counts[form]
+            -- if the source is higher priority and using it would put it under
+            -- the fill limit, don't use it as a source
+            if fill_dest == nil and diff > -AMOUNT_USED then
+                -- do nothing
+            else
+                fill_src = form
+                amount_spare = counts[form]
+            end
         end
         if fill_src ~= nil and fill_dest ~= nil then break end
     end