diff --git a/prototypes/ScheduledMerges.hs b/prototypes/ScheduledMerges.hs
index 8369cb06c..8e33893f1 100644
--- a/prototypes/ScheduledMerges.hs
+++ b/prototypes/ScheduledMerges.hs
@@ -287,7 +287,8 @@ newMerge tr level mergepolicy mergelast rs = do
                    mergeRunsSize = map Map.size rs
                  }
     assert (length rs `elem` [4, 5]) $
-      MergingRun mergepolicy mergelast <$> newSTRef (OngoingMerge debt rs r)
+      assert (mergeDebtLeft debt >= cost) $
+        MergingRun mergepolicy mergelast <$> newSTRef (OngoingMerge debt rs r)
   where
     cost = sum (map Map.size rs)
     -- How much we need to discharge before the merge can be guaranteed
@@ -299,7 +300,7 @@ newMerge tr level mergepolicy mergelast rs = do
     debt = newMergeDebt $ case mergepolicy of
              MergePolicyLevelling -> 4 * tieringRunSize (level-1)
                                        + levellingRunSize level
-             MergePolicyTiering   -> 4 * tieringRunSize (level-1)
+             MergePolicyTiering   -> length rs * tieringRunSize (level-1)
     -- deliberately lazy:
     r    = case mergelast of
              MergeMidLevel  ->                (mergek rs)
@@ -364,6 +365,10 @@ data MergeDebt =
 newMergeDebt :: Debt -> MergeDebt
 newMergeDebt d = MergeDebt 0 d
 
+mergeDebtLeft :: MergeDebt -> Int
+mergeDebtLeft (MergeDebt c d) =
+    assert (c < d) $ d - c
+
 -- | As credits are paid, debt is reduced in batches when sufficient credits have accumulated.
 data MergeDebtPaydown =
     -- | This remaining merge debt is fully paid off with credits.