diff --git a/lib/system/channels.nim b/lib/system/channels.nim
index d0294322a2..9c3cc93e0b 100644
--- a/lib/system/channels.nim
+++ b/lib/system/channels.nim
@@ -13,6 +13,9 @@
 ##
 ## **Note:** The current implementation of message passing is slow and does
 ## not work with cyclic data structures.
+  
+when not defined(NimString): 
+  {.error: "You must not import this module explicitly".}
 
 type
   pbytes = ptr array[0.. 0xffff, byte]
diff --git a/lib/system/threads.nim b/lib/system/threads.nim
index 7d74de92d2..104ca63c16 100644
--- a/lib/system/threads.nim
+++ b/lib/system/threads.nim
@@ -39,6 +39,9 @@
 ##    createThread(thr[i], threadFunc, (i*10, i*10+5))
 ##  joinThreads(thr)
   
+when not defined(NimString): 
+  {.error: "You must not import this module explicitly".}
+
 const
   maxRegisters = 256 # don't think there is an arch with more registers
   useStackMaskHack = false ## use the stack mask hack for better performance