diff --git a/lib/pure/streams.nim b/lib/pure/streams.nim
index 1ba0fb9aaf..e706f20160 100644
--- a/lib/pure/streams.nim
+++ b/lib/pure/streams.nim
@@ -226,8 +226,8 @@ proc ssWriteData(s: Stream, buffer: pointer, bufLen: int) =
   var s = StringStream(s)
   if bufLen <= 0:
     return
-  if s.pos + bufLen > s.data.len():
-    setLen(s.data, s.data.len + bufLen)
+  if s.pos + bufLen > s.data.len:
+    setLen(s.data, s.pos + bufLen)
   copyMem(addr(s.data[s.pos]), buffer, bufLen)
   inc(s.pos, bufLen)