diff --git a/lib/wabanex_web/schema/types/custom/date_range.ex b/lib/wabanex_web/schema/types/custom/date_range.ex
index c32d58e..45e2dd3 100644
--- a/lib/wabanex_web/schema/types/custom/date_range.ex
+++ b/lib/wabanex_web/schema/types/custom/date_range.ex
@@ -17,7 +17,7 @@ defmodule WabanexWeb.Schema.Types.Custom.DateRange do
   end
 
   @spec serialize_range(PgRanges.DateRange.t()) :: String.t()
-  @spec serialize_range(list(String.t())) :: String.t()
+  @spec serialize_range(list(DateTime.t() | nil)) :: String.t()
   defp serialize_range(%PgRanges.DateRange{lower: start_range, upper: end_range}),
     do: serialize_range([start_range, end_range])