[PATCH stable-2.16] Use only string value in error message

['Lisa Velden' via ganeti-devel]

Signed-off-by: Lisa Velden <vel...@google.com>
---
 src/Ganeti/WConfd/ConfigModifications.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/Ganeti/WConfd/ConfigModifications.hs 
b/src/Ganeti/WConfd/ConfigModifications.hs
index 9e66609..5b8c703 100644
--- a/src/Ganeti/WConfd/ConfigModifications.hs
+++ b/src/Ganeti/WConfd/ConfigModifications.hs
@@ -117,7 +117,7 @@ getAllIDs cs =
 
       instKeys = keysFromC . configInstances . csConfigData $ cs
       nodeKeys = keysFromC . configNodes . csConfigData $ cs
-      
+
       instValues = map uuidOf . valuesFromC
                  . configInstances . csConfigData $ cs
       nodeValues = map uuidOf . valuesFromC . configNodes . csConfigData $ cs
@@ -303,7 +303,7 @@ attachInstanceDiskChecks uuidInst uuidDisk idx' cs = do
   let insts = M.elems . fromContainer . configInstances . csConfigData $ cs
   forM_ insts (\inst' -> when (uuidDisk `elem` instDisks inst') . Bad
     . ReservationError $ printf "Disk %s already attached to instance %s"
-        uuidDisk (show $ instName inst))
+        uuidDisk (show $ fromMaybe "" (instName inst)))
 
 -- * Pure config modifications functions
 
-- 
2.6.0.rc2.230.g3dd15c0