diff --git a/src/ext/tinytest.c b/src/ext/tinytest.c
index cc054ad340..f6baeeb9a5 100644
--- a/src/ext/tinytest.c
+++ b/src/ext/tinytest.c
@@ -490,7 +490,7 @@ tinytest_format_hex_(const void *val_, unsigned long len)
 		return strdup("<allocation failure>");
 	cp = result;
 	for (i=0;i<len;++i) {
-		*cp++ = "0123456789ABCDEF"[val[i] >> 4];
+		*cp++ = "0123456789ABCDEF"[(val[i] >> 4)&0x0f];
 		*cp++ = "0123456789ABCDEF"[val[i] & 0x0f];
 	}
 	while (ellipses--)