diff --git a/C4/SIP/SIPServer.pm b/C4/SIP/SIPServer.pm
index d897a55a37..186373b3c8 100644
--- a/C4/SIP/SIPServer.pm
+++ b/C4/SIP/SIPServer.pm
@@ -323,8 +323,8 @@ sub raw_transport {
 
     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" );
diff --git a/C4/SIP/Sip.pm b/C4/SIP/Sip.pm
index c1450c8dff..c4cff7ee94 100644
--- a/C4/SIP/Sip.pm
+++ b/C4/SIP/Sip.pm
@@ -311,7 +311,15 @@ 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);