diff --git a/hw/ppc4xx_pci.c b/hw/ppc4xx_pci.c
index 2c69210..1bf785b 100644
--- a/hw/ppc4xx_pci.c
+++ b/hw/ppc4xx_pci.c
@@ -275,6 +275,10 @@ static void ppc4xx_pci_set_irq(void *opaque, int irq_num, int level)
     qemu_irq *pci_irqs = opaque;
 
     DPRINTF("%s: PCI irq %d\n", __func__, irq_num);
+    if (irq_num < 0) {
+        fprintf(stderr, "%s: PCI irq %d\n", __func__, irq_num);
+        return;
+    }
     qemu_set_irq(pci_irqs[irq_num], level);
 }