diff --git a/lib/system/sets.nim b/lib/system/sets.nim
index 4835bef1e7..83d18e5467 100644
--- a/lib/system/sets.nim
+++ b/lib/system/sets.nim
@@ -31,7 +31,7 @@ proc countBits64(n: uint64): int {.compilerproc, inline.} =
 
 proc cardSet(s: NimSet, len: int): int {.compilerproc, inline.} =
   var j = -1
-  when defined(x86):
+  when defined(x86) or defined(amd64):
     for i in countup(0, len - 8, 8):
       inc(result, countBits64((cast[ptr uint64](s[i].unsafeAddr))[]))
       j = i + 7