diff --git a/ponydos.asm b/ponydos.asm
index 4a9c574..d401f85 100644
--- a/ponydos.asm
+++ b/ponydos.asm
@@ -129,7 +129,7 @@ mainloop:
 ; in:
 ;  bx = width of input buffer
 ;  cx = width of rectangle
-;  dx = height of rectangle
+;  dx = height of rectangle (must be at least 1)
 ;  ds:si = beginning of source data
 ;  di = X
 ;  bp = Y