pi test compiles, but crashes randomly

tests/parallel/tpi.nim

@@ -0,0 +1,22 @@
+
+import strutils, math, threadpool
+
+proc term(k: float): float = 4 * math.pow(-1, k) / (2*k + 1)
+
+proc piU(n: int): float =
+  var ch = newSeq[Promise[float]](n+1)
+  for k in 0..n:
+    ch[k] = spawn term(float(k))
+  for k in 0..n:
+    result += ^ch[k]
+
+proc piS(n: int): float =
+  var ch = newSeq[float](n+1)
+  parallel:
+    for k in 0..ch.high:
+      ch[k] = spawn term(float(k))
+  for k in 0..ch.high:
+    result += ch[k]
+
+echo formatFloat(piU(5000))
+echo formatFloat(piS(5000))