diff --git a/doc/builtin/object_program b/doc/builtin/object_program
index 88f634f5600d3e9dd3e24dbf23014f1755c0b10b..f6a15bf3538590b361f593477cd1961a9067c69e 100644
--- a/doc/builtin/object_program
+++ b/doc/builtin/object_program
@@ -6,6 +6,7 @@ SYNTAX
 
 DESCRIPTION
 	This function returns the program from which o was cloned.
+	If o is not an object, or o was destructed zero is returned.
 
 KEYWORDS
 	object