diff --git a/cabal-install/Distribution/Client/InstallPlan.hs b/cabal-install/Distribution/Client/InstallPlan.hs
index 4f3aee7d8a6..f91761c048f 100644
--- a/cabal-install/Distribution/Client/InstallPlan.hs
+++ b/cabal-install/Distribution/Client/InstallPlan.hs
@@ -214,7 +214,7 @@ mkInstallPlan index indepGoals =
 internalError :: String -> a
 internalError msg = error $ "InstallPlan: internal error: " ++ msg
 
-instance (IsUnit ipkg, IsUnit srcpkg,
+instance (IsNode ipkg, Key ipkg ~ UnitId, IsNode srcpkg, Key srcpkg ~ UnitId,
           Binary ipkg, Binary srcpkg)
        => Binary (GenericInstallPlan ipkg srcpkg) where
     put GenericInstallPlan {