diff --git a/tests.c b/tests.c
index 632b36147..8f39d4026 100644
--- a/tests.c
+++ b/tests.c
@@ -46,18 +46,19 @@
 #include "ed25519.h"
 #include "script.h"
 
-uint8_t *fromhex(const char *str)
+#define FROMHEX_MAXLEN 256
+
+const uint8_t *fromhex(const char *str)
 {
-	static uint8_t buf[256];
-	uint8_t c;
-	for (size_t i = 0; i < strlen(str) / 2; i++) {
-		c = 0;
-		if (str[i*2] >= '0' && str[i*2] <= '9') c += (str[i*2] - '0') << 4;
-		if (str[i*2] >= 'a' && str[i*2] <= 'f') c += (10 + str[i*2] - 'a') << 4;
-		if (str[i*2] >= 'A' && str[i*2] <= 'F') c += (10 + str[i*2] - 'A') << 4;
-		if (str[i*2+1] >= '0' && str[i*2+1] <= '9') c += (str[i*2+1] - '0');
-		if (str[i*2+1] >= 'a' && str[i*2+1] <= 'f') c += (10 + str[i*2+1] - 'a');
-		if (str[i*2+1] >= 'A' && str[i*2+1] <= 'F') c += (10 + str[i*2+1] - 'A');
+	static uint8_t buf[FROMHEX_MAXLEN];
+	size_t len = strlen(str) / 2;
+	if (len > FROMHEX_MAXLEN) len = FROMHEX_MAXLEN;
+	for (size_t i = 0; i < len; i++) {
+		uint8_t c = 0;
+		if (str[i * 2] >= '0' && str[i*2] <= '9') c += (str[i * 2] - '0') << 4;
+		if ((str[i * 2] & ~0x20) >= 'A' && (str[i*2] & ~0x20) <= 'F') c += (10 + (str[i * 2] & ~0x20) - 'A') << 4;
+		if (str[i * 2 + 1] >= '0' && str[i * 2 + 1] <= '9') c += (str[i * 2 + 1] - '0');
+		if ((str[i * 2 + 1] & ~0x20) >= 'A' && (str[i * 2 + 1] & ~0x20) <= 'F') c += (10 + (str[i * 2 + 1] & ~0x20) - 'A');
 		buf[i] = c;
 	}
 	return buf;