diff --git a/eth/keyfile/keyfile.nim b/eth/keyfile/keyfile.nim
index da8a9ac..241e6ff 100644
--- a/eth/keyfile/keyfile.nim
+++ b/eth/keyfile/keyfile.nim
@@ -93,7 +93,10 @@ type
   KfResult*[T] = Result[T, KeyFileError]
 
 proc mapErrTo[T, E](r: Result[T, E], v: static KeyFileError): KfResult[T] =
-  r.mapErr(proc (e: E): KeyFileError = v)
+  if r.isOk:
+    ok(r.value)
+  else:
+    err(v)
 
 const
   SupportedHashes = [
diff --git a/eth/p2p/discoveryv5/protocol.nim b/eth/p2p/discoveryv5/protocol.nim
index 0ad9d74..ffde614 100644
--- a/eth/p2p/discoveryv5/protocol.nim
+++ b/eth/p2p/discoveryv5/protocol.nim
@@ -211,7 +211,7 @@ proc randomNodes*(d: Protocol, maxAmount: int): seq[Node] =
   d.routingTable.randomNodes(maxAmount)
 
 proc randomNodes*(d: Protocol, maxAmount: int,
-    pred: proc(x: Node): bool {.gcsafe, noSideEffect.}): seq[Node] =
+    pred: proc(x: Node): bool {.gcsafe, noSideEffect, raises:[Defect].}): seq[Node] =
   ## Get a `maxAmount` of random nodes from the local routing table with the
   ## `pred` predicate function applied as filter on the nodes selected.
   d.routingTable.randomNodes(maxAmount, pred)
diff --git a/eth/p2p/discoveryv5/routing_table.nim b/eth/p2p/discoveryv5/routing_table.nim
index 5ffb09c..f837b96 100644
--- a/eth/p2p/discoveryv5/routing_table.nim
+++ b/eth/p2p/discoveryv5/routing_table.nim
@@ -512,7 +512,7 @@ proc nodeToRevalidate*(r: RoutingTable): Node =
       return b.nodes[^1]
 
 proc randomNodes*(r: RoutingTable, maxAmount: int,
-    pred: proc(x: Node): bool {.gcsafe, noSideEffect.} = nil): seq[Node] =
+    pred: proc(x: Node): bool {.gcsafe, noSideEffect, raises:[Defect].} = nil): seq[Node] =
   ## Get a `maxAmount` of random nodes from the routing table with the `pred`
   ## predicate function applied as filter on the nodes selected.
   var maxAmount = maxAmount
diff --git a/eth/utp/growable_buffer.nim b/eth/utp/growable_buffer.nim
index 093ce5b..a34d479 100644
--- a/eth/utp/growable_buffer.nim
+++ b/eth/utp/growable_buffer.nim
@@ -21,6 +21,9 @@ type
     items: seq[Option[A]]
     mask: uint32
 
+when not defined(nimHasEffectsOfs):
+  template effectsOf(f: untyped) {.pragma.}
+
 # provided size will always be adjusted to next power of two
 proc init*[A](T: type GrowableCircularBuffer[A], size: uint32 = 16): T =
   let powOfTwoSize = nextPowerOfTwo(int(size))
@@ -44,11 +47,13 @@ proc delete*[A](buff: var GrowableCircularBuffer[A], i: uint32) =
 proc hasKey*[A](buff: GrowableCircularBuffer[A], i: uint32): bool =
   buff.get(i).isSome()
 
-proc exists*[A](buff: GrowableCircularBuffer[A], i: uint32, check: proc (x: A): bool): bool =
+proc exists*[A](buff: GrowableCircularBuffer[A], i: uint32,
+                check: proc (x: A): bool): bool {.gcsafe, effectsOf: check.} =
   let maybeElem = buff.get(i)
   if (maybeElem.isSome()):
     let elem = maybeElem.unsafeGet()
-    check(elem)
+    {.gcsafe.}:
+      check(elem)
   else:
     false