diff --git a/C4/SIP/SIPServer.pm b/C4/SIP/SIPServer.pm
index 96c1809fbe..0038271ca6 100644
--- a/C4/SIP/SIPServer.pm
+++ b/C4/SIP/SIPServer.pm
@@ -313,8 +313,9 @@ sub raw_transport {
     Koha::Logger->put_mdc( "peeraddr",  $self->{server}->{peeraddr} );
 
     siplog("LOG_DEBUG", "raw_transport: uname/inst: '%s/%s'",
-        $self->{account}->{id},
-        $self->{account}->{institution});
+        $self->{account}->{id}          // 'undef',
+        $self->{account}->{institution} // 'undef'
+    );
     if (! $self->{account}->{id}) {
         siplog("LOG_ERR","Login failed shutting down");
         return;
diff --git a/C4/SIP/Sip.pm b/C4/SIP/Sip.pm
index fa65951bd9..8d27f64af1 100644
--- a/C4/SIP/Sip.pm
+++ b/C4/SIP/Sip.pm
@@ -231,7 +231,15 @@ sub siplog {
       : $level eq 'LOG_WARNING' ? 'warn'
       :                           'error';
 
-    my $message = @args ? sprintf($mask, @args) : $mask;
+    my $message;
+    if (@args) {
+
+        # Replace undefined values with 'undef' to prevent sprintf warnings
+        my @safe_args = map { defined($_) ? $_ : 'undef' } @args;
+        $message = sprintf( $mask, @safe_args );
+    } else {
+        $message = $mask;
+    }
 
     $message = remove_password_from_message($message);