lib/PublicInbox/Git.pm | 14 ++++++++------
 t/git.t                | 21 +++++++++++++++++++++

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 9059584086ca57ecfb3411cb2cae12b87cc9496b..af3a5712cf79dec86c90e4f92eb9a2df87a3683e 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -148,16 +148,18 @@ }
 
 sub _cat_async_step ($$$) {
 	my ($self, $inflight, $in) = @_;
-	my $cb = shift @$inflight or die 'BUG: inflight empty';
+	my $pair = shift @$inflight or die 'BUG: inflight empty';
+	my ($cb, $arg) = @$pair;
 	local $/ = "\n";
 	my $head = $in->getline;
-	return eval { $cb->(undef) } if $head =~ / missing$/;
+	$head =~ / missing$/ and return
+		eval { $cb->(undef, undef, undef, undef, $arg) };
 
 	$head =~ /^([0-9a-f]{40}) (\S+) ([0-9]+)$/ or
 		fail($self, "Unexpected result from async git cat-file: $head");
 	my ($oid_hex, $type, $size) = ($1, $2, $3 + 0);
 	my $bref = read_cat_in_full($self, $in, $size);
-	eval { $cb->($bref, $oid_hex, $type, $size) };
+	eval { $cb->($bref, $oid_hex, $type, $size, $arg) };
 }
 
 sub cat_async_wait ($) {
@@ -319,15 +321,15 @@ 	die 'BUG: already in async' if $self->{inflight};
 	$self->{inflight} = [];
 }
 
-sub cat_async ($$$) {
-	my ($self, $oid, $cb) = @_;
+sub cat_async ($$$;$) {
+	my ($self, $oid, $cb, $arg) = @_;
 	my $inflight = $self->{inflight} or die 'BUG: not in async';
 	if (scalar(@$inflight) >= MAX_INFLIGHT) {
 		_cat_async_step($self, $inflight, $self->{in});
 	}
 
 	$self->{out}->print($oid, "\n") or fail($self, "write error: $!");
-	push @$inflight, $cb;
+	push(@$inflight, [ $cb, $arg ]);
 }
 
 sub commit_title ($$) {
diff --git a/t/git.t b/t/git.t
index d4694f44c2d98393458a856c7b7dabe502c9acdf..6cfadd085f20d82c46c47c015da870b1e74476e9 100644
--- a/t/git.t
+++ b/t/git.t
@@ -33,6 +33,27 @@ 	is($x[2], length($$raw), 'length matches');
 
 	is(${$gcf->cat_file($f)}, $$raw, 'not broken after failures');
 	is(${$gcf->cat_file($f)}, $$raw, 'not broken after partial read');
+
+	my $oid = $x[0];
+	my $arg = { 'foo' => 'bar' };
+	my $res = [];
+	my $missing = [];
+	$gcf->cat_async_begin;
+	$gcf->cat_async($oid, sub {
+		my ($bref, $oid_hex, $type, $size, $arg) = @_;
+		$res = [ @_ ];
+	}, $arg);
+	$gcf->cat_async('non-existent', sub {
+		my ($bref, $oid_hex, $type, $size, $arg) = @_;
+		$missing = [ @_ ];
+	}, $arg);
+	$gcf->cat_async_wait;
+	my ($bref, $oid_hex, $type, $size, $arg_res) = @$res;
+	is_deeply([$oid_hex, $type, $size], \@x, 'got expected header');
+	is($arg_res, $arg, 'arg passed to cat_async');
+	is_deeply($raw, $bref, 'blob result matches');
+	is_deeply($missing, [ undef, undef, undef, undef, $arg],
+		'non-existent blob gives expected result');
 }
 
 if (1) {