diff --git a/tests/test6.in b/tests/test6.in
index dffe5e1a9..0ea772bb9 100644
--- a/tests/test6.in
+++ b/tests/test6.in
@@ -67,6 +67,11 @@ echo "Expect -AQ -AW:" (complete -C'TestExclusive -A' | sort | string join ' ')
 echo "Expect no output 1:" (complete -C'TestExclusive -Q')
 echo "Expect no output 2:" (complete -C'TestExclusive -W')
 
+# Test for optional arguments, like cp's --backup
+complete -c TestOptionalArgument -l backup -f -a 'none all simple'
+echo "Expect --backup --backup=:" (complete -C'TestOptionalArgument -' | sort | string join ' ')
+echo "Expect --backup=all  --backup=none  --backup=simple:" (complete -C'TestOptionalArgument --backup=' | sort | string join ' ')
+
 # Test that directory completions work correctly
 if begin; rm -rf test6.tmp.dir; and mkdir test6.tmp.dir; end
     pushd test6.tmp.dir
diff --git a/tests/test6.out b/tests/test6.out
index 81c2684a7..6e21926d2 100644
--- a/tests/test6.out
+++ b/tests/test6.out
@@ -38,6 +38,8 @@ Expect -A -Q -W: -A -Q -W
 Expect -AQ -AW: -AQ -AW
 Expect no output 1:
 Expect no output 2:
+Expect --backup --backup=: --backup --backup=
+Expect --backup=all  --backup=none  --backup=simple: --backup=all --backup=none --backup=simple
 implicit cd complete works
 no implicit cd complete after 'command'
 PATH does not cause incorrect implicit cd