diff --git a/retropikzel/pstk.scm b/retropikzel/pstk.scm
index d4259cd..f281045 100644
--- a/retropikzel/pstk.scm
+++ b/retropikzel/pstk.scm
@@ -38,8 +38,8 @@
           (open-output-pipe output-path))))
 
 (define *wish-program* "tclsh")
-(define *wish-debug-input* #t)
-(define *wish-debug-output* #t)
+(define *wish-debug-input* (if (get-environment-variable "PSTK_DEBUG") #t #f))
+(define *wish-debug-output* (if (get-environment-variable "PSTK_DEBUG") #t #f))
 
 (define *use-keywords?*
   (cond-expand (chicken #t)
diff --git a/retropikzel/pstk/README.md b/retropikzel/pstk/README.md
index 70dec60..4cc8473 100644
--- a/retropikzel/pstk/README.md
+++ b/retropikzel/pstk/README.md
@@ -12,6 +12,7 @@ The library now uses (retropikzel named-pipes) library, which uses (foreign c)
 underneath. It should work on any implementation those libraries work on.
 
 
+Set PSTK_DEBUG environment variable to see debug output.
 
 ### 1.1. Simple Example
 
diff --git a/retropikzel/pstk/VERSION b/retropikzel/pstk/VERSION
index 21e8796..ee90284 100644
--- a/retropikzel/pstk/VERSION
+++ b/retropikzel/pstk/VERSION
@@ -1 +1 @@
-1.0.3
+1.0.4