diff --git a/tests/memory-block-merging/coalescing/map/map15.fut b/tests/memory-block-merging/coalescing/map/map15.fut
index ed3059dc0a..42c443a1d1 100644
--- a/tests/memory-block-merging/coalescing/map/map15.fut
+++ b/tests/memory-block-merging/coalescing/map/map15.fut
@@ -3,11 +3,18 @@
 -- original consumed 'u'.  Each thread _does_ need a distinct copy of
 -- 'u', and they cannot share memory.
 -- ==
+-- input { [[1.0f32, 2.0f32, 3.0f32], [4.0f32, 5.0f32, 6.0f32], [7.0f32, 8.0f32, 9.0f32]] [42.0f32, 1337.0f32] }
+-- output { [[[1.0f32, -40.0f32, 1683.0f32],
+--           [4.0f32, -163.0f32, 6852.0f32],
+--           [7.0f32, -286.0f32, 12021.0f32]],
+--          [[1.0f32, -1335.0f32, 1784898.0f32],
+--           [4.0f32, -5343.0f32, 7143597.0f32],
+--           [7.0f32, -9351.0f32, 1.2502296e7f32]]] }
 -- structure gpu-mem { Alloc 3 }
 
 def tridagSeq [m] (y: *[m]f32) (beta: f32) =
     loop y
-      for i < m do
+      for i < m-1 do
         let i    = i + 1
         let y[i] = y[i] - beta*y[i-1]
         in  y