diff --git a/man2/mmap.2 b/man2/mmap.2
index 47c314865..34c0cec05 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -59,7 +59,7 @@ The starting address for the new mapping is specified in
 .IR addr .
 The
 .I length
-argument specifies the length of the mapping.
+argument specifies the length of the mapping (which must be greater than 0).
 .PP
 If
 .I addr