diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 029c08b..f621fc0 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -10376,6 +10376,8 @@
 		character in window {winid} at buffer line {lnum} and virtual
 		column {col}.
 
+		If buffer line {lnum} is an empty line, 0 is returned.
+
 		If {col} is greater than the last virtual column in line
 		{lnum}, then the byte index of the character at the last
 		virtual column is returned.