diff --git a/runtime/macros/less.sh b/runtime/macros/less.sh
index 484c714..e29958f 100755
--- a/runtime/macros/less.sh
+++ b/runtime/macros/less.sh
@@ -1,16 +1,24 @@
 #!/bin/sh
 # Shell script to start Vim with less.vim.
-# Read stdin if no arguments were given.
+# Read stdin if no arguments were given and stdin was redirected.
 
 if test -t 1; then
- if test $# = 0; then
-   vim --cmd 'let no_plugin_maps = 1' -c 'runtime! macros/less.vim' -
+  if test $# = 0; then
+    if test -t 0; then
+      echo "Missing filename" 1>&2
+      exit
+    fi
+    vim --cmd 'let no_plugin_maps = 1' -c 'runtime! macros/less.vim' -
   else
-   vim --cmd 'let no_plugin_maps = 1' -c 'runtime! macros/less.vim' "$@"
+    vim --cmd 'let no_plugin_maps = 1' -c 'runtime! macros/less.vim' "$@"
   fi
 else
   # Output is not a terminal, cat arguments or stdin
   if test $# = 0; then
+    if test -t 0; then
+      echo "Missing filename" 1>&2
+      exit
+    fi
     cat
   else
     cat "$@"