diff --git a/hledger-lib/Hledger/Reports/ReportTypes.hs b/hledger-lib/Hledger/Reports/ReportTypes.hs
index 9b6508f21..301bf331a 100644
--- a/hledger-lib/Hledger/Reports/ReportTypes.hs
+++ b/hledger-lib/Hledger/Reports/ReportTypes.hs
@@ -148,7 +148,7 @@ data CompoundPeriodicReport a b = CompoundPeriodicReport
   , cbrDates      :: [DateSpan]
   , cbrSubreports :: [(String, PeriodicReport a b, Bool)]
   , cbrTotals     :: PeriodicReportRow () b
-  } deriving (Show, Generic, ToJSON)
+  } deriving (Show, Functor, Generic, ToJSON)
 
 -- | Description of one subreport within a compound balance report.
 -- Part of a "CompoundBalanceCommandSpec", but also used in hledger-lib.