diff --git a/lib/Test/IO/Capture.pm b/lib/Test/IO/Capture.pm
index cab799a..2991521 100644
--- a/lib/Test/IO/Capture.pm
+++ b/lib/Test/IO/Capture.pm
@@ -3,13 +3,13 @@ use IO::Capture::Simple;
 
 unit module Test::IO::Capture;
 
-sub prints-stdout-ok (Callable $code, Str $expected, $reason = '')
+sub prints-stdout-ok (Callable $code, Str() $expected, $reason = '')
 is export {
     my $out = capture_stdout $code;
     is $out, $expected, $reason;
 }
 
-sub prints-stderr-ok (Callable $code, Str $expected, $reason = '')
+sub prints-stderr-ok (Callable $code, Str() $expected, $reason = '')
 is export {
     my $out = capture_stderr $code;
     is $out, $expected, $reason;