diff --git a/apps/explorer/lib/explorer/chain/import.ex b/apps/explorer/lib/explorer/chain/import.ex
index 470a93851a..c9be328246 100644
--- a/apps/explorer/lib/explorer/chain/import.ex
+++ b/apps/explorer/lib/explorer/chain/import.ex
@@ -1047,7 +1047,7 @@ defmodule Explorer.Chain.Import do
         ]
       )
 
-    {sql, parameters} = SQL.to_sql(:all, Repo, query)
+    {sql, parameters} = SQL.to_sql(:all, Repo, query) |> IO.inspect()
 
     {:ok, %Postgrex.Result{columns: ["uncle_hash", "hash"], command: :insert, rows: rows}} =
       SQL.query(
diff --git a/apps/explorer/test/explorer/chain/import_test.exs b/apps/explorer/test/explorer/chain/import_test.exs
index 59145714cd..0bf54ffa4f 100644
--- a/apps/explorer/test/explorer/chain/import_test.exs
+++ b/apps/explorer/test/explorer/chain/import_test.exs
@@ -1629,5 +1629,17 @@ defmodule Explorer.Chain.ImportTest do
 
       assert DateTime.compare(timestamp, timestamp_before) == :eq
     end
+
+    # https://github.com/poanetwork/blockscout/issues/850 regression test
+    test "derive_transaction_forks returns errors" do
+      _pending_transaction = insert(:transaction)
+
+      {:error, :derive_transaction_forks, %Postgrex.Error{postgres: %{code: :not_null_violation, column: "index"}}, _} =
+        Import.all(%{
+          blocks: %{
+            params: []
+          }
+        })
+    end
   end
 end