diff --git a/autoload/diff.vim b/autoload/diff.vim
index 7d8a983..8d5d1e0 100644
--- a/autoload/diff.vim
+++ b/autoload/diff.vim
@@ -17,10 +17,15 @@ function! diff#run_diff(realtime, use_external_grep)
   endif
 
   if a:use_external_grep && s:grep_available
-    " grep exits with 1 when no matches are found.  However we want to treat
-    " non-matches as non-erroneous behaviour; so we OR the command with one
+    let cmd .= s:grep_command
+  endif
+
+  if (a:use_external_grep && s:grep_available) || a:realtime
+    " grep exits with 1 when no matches are found; diff exits with 1 when
+    " differences are found.  However we want to treat non-matches and
+    " differences as non-erroneous behaviour; so we OR the command with one
     " which always returns true.
-    let cmd .= s:grep_command . ' || true'
+    let cmd.= ' || true'
   endif
 
   let cmd .= '))'