diff --git a/lib/pure/unittest.nim b/lib/pure/unittest.nim
index cfb7622585..389a083c6b 100644
--- a/lib/pure/unittest.nim
+++ b/lib/pure/unittest.nim
@@ -109,7 +109,7 @@ import std/private/since
 import std/exitprocs
 
 when defined(nimPreviewSlimSystem):
-  import std/assertions
+  import std/[assertions, syncio]
 
 import std/[macros, strutils, streams, times, sets, sequtils]