diff --git a/lib/Plack/Middleware/StackTrace.pm b/lib/Plack/Middleware/StackTrace.pm
index 9c02c109..d0821e53 100644
--- a/lib/Plack/Middleware/StackTrace.pm
+++ b/lib/Plack/Middleware/StackTrace.pm
@@ -55,7 +55,8 @@ sub call {
         $trace = $caught_trace if $caught_trace;
     }
 
-    if ($trace && ($caught || ($self->force && ref $res eq 'ARRAY' && $res->[0] == 500)) ) {
+    # Use ref $trace to avoid calling as_string twice when it's not necessary
+    if (ref $trace && ($caught || ($self->force && ref $res eq 'ARRAY' && $res->[0] == 500)) ) {
         my $text = $trace->as_string;
         my $html = $trace->as_html;
         $env->{'plack.stacktrace.text'} = $text;