diff --git a/c_src/esqlite3_nif.c b/c_src/esqlite3_nif.c
index 50cc3f7..361c425 100644
--- a/c_src/esqlite3_nif.c
+++ b/c_src/esqlite3_nif.c
@@ -943,7 +943,7 @@ esqlite_backup_step(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[])
 
     int rc = sqlite3_backup_step(backup->backup, n_page);
     if(rc == SQLITE_DONE) {
-        return make_atom(env, "done");
+        return make_atom(env, "$done");
     }  
 
     if(rc != SQLITE_OK) {
diff --git a/src/esqlite3.erl b/src/esqlite3.erl
index ea462fa..8a4cdf0 100644
--- a/src/esqlite3.erl
+++ b/src/esqlite3.erl
@@ -430,22 +430,28 @@ backup_init(#esqlite3{db=Dest}, DestName, #esqlite3{db=Src}, SrcName) ->
     end.
 
 %% @doc Release the resources held by the backup.
--spec backup_finish(esqlite3_backup()) -> ok | {error, _}.
+-spec backup_finish(Backup) -> Result when
+      Backup :: esqlite3_backup(),
+      Result :: ok | error().
 backup_finish(#esqlite3_backup{backup=Backup}) ->
     esqlite3_nif:backup_finish(Backup).
 
 %% @doc Do a backup step. 
--spec backup_step(esqlite3_backup(), integer()) -> ok | {error, _}.
+-spec backup_step(esqlite3_backup(), integer()) -> ok | '$done' | error().
 backup_step(#esqlite3_backup{backup=Backup}, NPage) ->
     esqlite3_nif:backup_step(Backup, NPage).
 
 %% @doc Get the remaining number of pages which need to be backed up.
--spec backup_remaining(esqlite3_backup()) -> pos_integer().
+-spec backup_remaining(Backup) -> Remaining when
+      Backup :: esqlite3_backup(),
+      Remaining :: integer().
 backup_remaining(#esqlite3_backup{backup=Backup}) ->
     esqlite3_nif:backup_remaining(Backup).
 
 %% @doc Get the remaining number of pages which need to be backed up.
--spec backup_pagecount(esqlite3_backup()) -> pos_integer().
+-spec backup_pagecount(Backup) -> Pagecount when
+      Backup :: esqlite3_backup(),
+      Pagecount :: integer().
 backup_pagecount(#esqlite3_backup{backup=Backup}) ->
     esqlite3_nif:backup_pagecount(Backup).
 
diff --git a/src/esqlite3_nif.erl b/src/esqlite3_nif.erl
index e6a1511..e341f82 100644
--- a/src/esqlite3_nif.erl
+++ b/src/esqlite3_nif.erl
@@ -224,25 +224,37 @@ column_decltypes(_Stmt) ->
 
 
 %% @doc Initialize a backup procedure of a database.
-%    erlang:nif_error(nif_library_not_loaded).
 -spec backup_init(Destination, DestinationName, Source, SourceName) -> InitResult when
       Destination :: esqlite3_ref(),
       DestinationName :: iodata(),
       Source :: esqlite3_ref(),
       SourceName :: iodata(),
-      InitResult :: {ok, esqlite3_backup_ref()} | {error, _}.
+      InitResult :: {ok, esqlite3_backup_ref()} | error().
 backup_init(_Dest, _DestName, _Src, _SrcName) ->
     erlang:nif_error(nif_library_not_loaded).
 
+-spec backup_remaining(Backup) -> Remaining when
+      Backup :: esqlite3_backup_ref(),
+      Remaining :: integer().
 backup_remaining(_Backup) ->
     erlang:nif_error(nif_library_not_loaded).
 
+-spec backup_pagecount(Backup) -> Pagecount when
+      Backup :: esqlite3_backup_ref(),
+      Pagecount :: integer().
 backup_pagecount(_Backup) ->
     erlang:nif_error(nif_library_not_loaded).
 
+-spec backup_step(Backup, NPage) -> Result when
+      Backup :: esqlite3_backup_ref(),
+      NPage :: integer(),
+      Result :: ok | '$done' | error().
 backup_step(_Backup, _PageCount) ->
     erlang:nif_error(nif_library_not_loaded).
 
+-spec backup_finish(Backup) -> Result when
+      Backup :: esqlite3_backup_ref(),
+      Result :: ok | error().
 backup_finish(_Backup) ->
     erlang:nif_error(nif_library_not_loaded).
 
diff --git a/test/esqlite_test.erl b/test/esqlite_test.erl
index 5238204..6cf8de7 100644
--- a/test/esqlite_test.erl
+++ b/test/esqlite_test.erl
@@ -430,7 +430,7 @@ backup_test() ->
     0 = esqlite3:backup_remaining(Backup),
     0 = esqlite3:backup_pagecount(Backup),
 
-    done = esqlite3:backup_step(Backup, 1),
+    '$done' = esqlite3:backup_step(Backup, 1),
 
     cleanup(),
 
@@ -467,7 +467,7 @@ backup1_test() ->
     27 = esqlite3:backup_pagecount(Backup),
 
     %% Do all the remaining pages.
-    done = esqlite3:backup_step(Backup, -1),
+    '$done' = esqlite3:backup_step(Backup, -1),
 
     0 = esqlite3:backup_remaining(Backup),
     27 = esqlite3:backup_pagecount(Backup),