diff --git a/hw/pci-host/uninorth.c b/hw/pci-host/uninorth.c
index 53f2b59ae8..f0144eb7b0 100644
--- a/hw/pci-host/uninorth.c
+++ b/hw/pci-host/uninorth.c
@@ -92,7 +92,10 @@ static uint32_t unin_get_config_reg(uint32_t reg, uint32_t addr)
         uint32_t slot, func;
 
         /* Grab CFA0 style values */
-        slot = ffs(reg & 0xfffff800) - 1;
+        slot = ctz32(reg & 0xfffff800);
+        if (slot == 32) {
+            slot = -1; /* XXX: should this be 0? */
+        }
         func = (reg >> 8) & 7;
 
         /* ... and then convert them to x86 format */