diff --git a/lib/io b/lib/io
index 95cc5fa4e076ecb918c3e616962715ea242e49fd..aae80723f67b8a3312f2bb317ab766023f4d5372 100755
--- a/lib/io
+++ b/lib/io
@@ -127,7 +127,7 @@ function abort() {
 function warn() {
   @required [string] message
 
-  COLUMNS=100 @go.printf "${UI_Color_Magenta}Warning: %b%s${UI_Color_Default}\n" "$message" >&"${STDERR}"
+  COLUMNS=100 @go.printf "${UI_Color_Magenta}Warning: %b%s${UI_Color_Default}\n" "$message" >&"$"{STDERR:-2}
 }
 
 function set_standard_outputs() {