diff --git a/config/config.nims b/config/config.nims
index 03daa59702..24c7901681 100644
--- a/config/config.nims
+++ b/config/config.nims
@@ -2,3 +2,4 @@
 
 when defined(nimHasCppDefine):
   cppDefine "errno"
+  cppDefine "unix"