diff --git a/lib/sbi/riscv_asm.c b/lib/sbi/riscv_asm.c
index 4c24a513..d7b9b2b2 100644
--- a/lib/sbi/riscv_asm.c
+++ b/lib/sbi/riscv_asm.c
@@ -213,6 +213,9 @@ static unsigned long ctz(unsigned long x)
 {
 	unsigned long ret = 0;
 
+	if (x == 0)
+		return 8 * sizeof(x);
+
 	while (!(x & 1UL)) {
 		ret++;
 		x = x >> 1;