diff --git a/src/Compiler/Scheme/Common.idr b/src/Compiler/Scheme/Common.idr
index 05362a1..1774212 100644
--- a/src/Compiler/Scheme/Common.idr
+++ b/src/Compiler/Scheme/Common.idr
@@ -425,7 +425,7 @@ parameters (schExtPrim : Int -> ExtPrim -> List NamedCExp -> Core String,
   schExtCommon i VoidElim [_, _]
       = pure "(display \"Error: Executed 'void'\")"
   schExtCommon i SysOS []
-      = pure $ show os
+      = pure $ "(blodwen-os)"
   schExtCommon i (Unknown n) args
       = throw (InternalError ("Can't compile unknown external primitive " ++ show n))
   schExtCommon i prim args
diff --git a/support/chez/support.ss b/support/chez/support.ss
index 3434818..5e36f73 100644
--- a/support/chez/support.ss
+++ b/support/chez/support.ss
@@ -1,3 +1,10 @@
+(define (blodwen-os)
+  (case (machine-type)
+    [(i3le ti3le a6le ta6le) "unix"]
+    [(i3osx ti3osx a6osx ta6osx) "darwin"]
+    [(i3nt ti3nt a6nt ta6nt) "windows"]
+    [else "unknown"]))
+
 (define blodwen-read-args (lambda (desc)
   (case (vector-ref desc 0)
     ((0) '())