diff --git a/tests/codex/testerasure.nim b/tests/codex/testerasure.nim
index 41732a1a..fa034001 100644
--- a/tests/codex/testerasure.nim
+++ b/tests/codex/testerasure.nim
@@ -1,4 +1,5 @@
 import std/sequtils
+import std/sugar
 
 import pkg/asynctest
 import pkg/chronos
@@ -10,6 +11,7 @@ import pkg/codex/manifest
 import pkg/codex/stores
 import pkg/codex/blocktype as bt
 import pkg/codex/rng
+import pkg/codex/utils
 
 import ./helpers
 
@@ -42,7 +44,7 @@ asyncchecksuite "Erasure encode/decode":
 
     check:
       encoded.blocksCount mod (buffers + parity) == 0
-      encoded.rounded == (manifest.blocksCount + (buffers - (manifest.blocksCount mod buffers)))
+      encoded.rounded == roundUp(manifest.blocksCount, buffers)
       encoded.steps == encoded.rounded div buffers
 
     return encoded
@@ -55,14 +57,14 @@ asyncchecksuite "Erasure encode/decode":
     let encoded = await encode(buffers, parity)
 
     var
-      column = rng.rand((encoded.blocksCount - 1) div encoded.steps) # random column
+      column = rng.rand((encoded.blocksCount div encoded.steps) - 1) # random column
       dropped: seq[int]
 
     for _ in 0..<encoded.ecM:
       dropped.add(column)
       (await store.delBlock(encoded.treeCid, column)).tryGet()
       (await store.delBlock(manifest.treeCid, column)).tryGet()
-      column.inc(encoded.steps - 1)
+      column = (column + encoded.steps) mod encoded.blocksCount # wrap around
 
     var
       decoded = (await erasure.decode(encoded)).tryGet()
@@ -73,8 +75,9 @@ asyncchecksuite "Erasure encode/decode":
       decoded.blocksCount == encoded.originalBlocksCount
 
     for d in dropped:
-      let present = await store.hasBlock(manifest.treeCid, d)
-      check present.tryGet()
+      if d < manifest.blocksCount: # we don't support returning parity blocks yet
+        let present = await store.hasBlock(manifest.treeCid, d)
+        check present.tryGet()
 
   test "Should not tolerate losing more than M data blocks in a single random column":
     const
@@ -84,14 +87,14 @@ asyncchecksuite "Erasure encode/decode":
     let encoded = await encode(buffers, parity)
 
     var
-      column = rng.rand((encoded.blocksCount - 1) div encoded.steps) # random column
+      column = rng.rand((encoded.blocksCount div encoded.steps) - 1) # random column
       dropped: seq[int]
 
     for _ in 0..<encoded.ecM + 1:
       dropped.add(column)
       (await store.delBlock(encoded.treeCid, column)).tryGet()
       (await store.delBlock(manifest.treeCid, column)).tryGet()
-      column.inc(encoded.steps)
+      column = (column + encoded.steps) mod encoded.blocksCount # wrap around
 
     var
       decoded: Manifest
@@ -144,7 +147,7 @@ asyncchecksuite "Erasure encode/decode":
       blocks: seq[int]
       offset = 0
 
-    while offset < encoded.steps - 1:
+    while offset < encoded.steps:
       let
         blockIdx = toSeq(countup(offset, encoded.blocksCount - 1, encoded.steps))
 
@@ -177,7 +180,8 @@ asyncchecksuite "Erasure encode/decode":
 
     let encoded = await encode(buffers, parity)
 
-    for b in 0..<encoded.steps * encoded.ecM:
+    # loose M original (systematic) symbols/blocks
+    for b in 0..<(encoded.steps * encoded.ecM):
       (await store.delBlock(encoded.treeCid, b)).tryGet()
       (await store.delBlock(manifest.treeCid, b)).tryGet()
 
@@ -192,9 +196,14 @@ asyncchecksuite "Erasure encode/decode":
       buffers = 20
       parity = 10
 
-    let encoded = await encode(buffers, parity)
+    let
+      encoded = await encode(buffers, parity)
+      blocks = collect:
+        for i in 0..encoded.blocksCount:
+          i
 
-    for b in (encoded.blocksCount - encoded.steps * encoded.ecM)..<encoded.blocksCount:
+    # loose M parity (all!) symbols/blocks from the dataset
+    for b in blocks[^(encoded.steps * encoded.ecM)..^1]:
       (await store.delBlock(encoded.treeCid, b)).tryGet()
       (await store.delBlock(manifest.treeCid, b)).tryGet()