diff --git a/src/hanoidb_bloom.erl b/src/hanoidb_bloom.erl
index 154bc9d..8d385d7 100644
--- a/src/hanoidb_bloom.erl
+++ b/src/hanoidb_bloom.erl
@@ -196,7 +196,10 @@ bitmask_new(LogN) ->
 bitmask_set(I, BM) ->
     case element(1,BM) of
         array -> bitarray_set(I, BM);
-        dense_bitmap_ets -> hanoidb_dense_bitmap:set(I, BM)
+        dense_bitmap_ets -> hanoidb_dense_bitmap:set(I, BM);
+        dense_bitmap ->
+            %% Surprise - we need to mutate a built representation:
+            hanoidb_dense_bitmap:set(I, hanoidb_dense_bitmap:unbuild(BM))
     end.
 
 %%% Convert to external form.
diff --git a/src/hanoidb_dense_bitmap.erl b/src/hanoidb_dense_bitmap.erl
index 3bf25f4..300ccf8 100644
--- a/src/hanoidb_dense_bitmap.erl
+++ b/src/hanoidb_dense_bitmap.erl
@@ -1,6 +1,6 @@
 -module(hanoidb_dense_bitmap).
 
--export([new/1, set/2, build/1, member/2]).
+-export([new/1, set/2, build/1, unbuild/1, member/2]).
 -define(BITS_PER_CELL, 32).
 
 -define(REPR_NAME, dense_bitmap).
@@ -10,7 +10,6 @@ new(N) ->
     Width = 1 + (N-1) div ?BITS_PER_CELL,
     Value = erlang:make_tuple(Width+1, 0, [{1,?REPR_NAME}]),
     ets:insert(Tab, Value),
-    %io:format("DB| create(): ~p of width ~p\n", [Tab, Width]),
     {dense_bitmap_ets, N, Width, Tab}.
 
 %% Set a bit.
@@ -27,6 +26,11 @@ build({dense_bitmap_ets, _, _, Tab}) ->
     ets:delete(Tab),
     Row.
 
+unbuild(Row) when element(1,Row)==?REPR_NAME ->
+    Tab = ets:new(dense_bitmap, [private, set]),
+    ets:insert(Tab, Row),
+    {dense_bitmap_ets, undefined, undefined, Tab}.
+
 member(I, Row) when element(1,Row)==?REPR_NAME ->
     Cell = 2 + I div ?BITS_PER_CELL,
     BitInCell = I rem ?BITS_PER_CELL,