diff --git a/lib/libc/snprintf.c b/lib/libc/snprintf.c
index 1661143..7141091 100644
--- a/lib/libc/snprintf.c
+++ b/lib/libc/snprintf.c
@@ -40,6 +40,12 @@
 	unsigned int rem;
 	char ascii_a = capitalise ? 'A' : 'a';
 
+	if (radix < 10) {
+		ERROR("snprintf: unsupported radix '%d'.", radix);
+		plat_panic_handler();
+		assert(0); /* Unreachable */
+	}
+
 	do {
 		rem = unum % radix;
 		if (rem < 10U) {