diff --git a/lib/pure/streams.nim b/lib/pure/streams.nim
index 88ac626a67..38e91fee48 100644
--- a/lib/pure/streams.nim
+++ b/lib/pure/streams.nim
@@ -103,7 +103,7 @@ proc readData*(s: Stream, buffer: pointer, bufLen: int): int =
 
 proc readAll*(s: Stream): string =
   ## Reads all available data.
-  let bufferSize = 1000
+  const bufferSize = 1000
   result = newString(bufferSize)
   var r = 0
   while true: