diff --git a/FU/MultipartFormData.pm b/FU/MultipartFormData.pm
index 8441740..7c4f467 100644
--- a/FU/MultipartFormData.pm
+++ b/FU/MultipartFormData.pm
@@ -77,7 +77,7 @@ sub syswrite($o, $fh) {
 
 sub save($o, $fn) {
     open my $F, '>', $fn or confess "Error opening '$fn': $!";
-    $o->syswrite($F) or confess "Error writing to '$fn': $!";
+    defined $o->syswrite($F) or confess "Error writing to '$fn': $!";
 }
 
 sub describe($o) {