diff --git a/f3probe.c b/f3probe.c
index 15b0d2e..0476123 100644
--- a/f3probe.c
+++ b/f3probe.c
@@ -39,7 +39,7 @@ static struct argp_option options[] = {
 		"Enable debuging with a regular file",	1},
 	{"debug-fake-size",	'f',	"SIZE_GB",	OPTION_HIDDEN,
 		"Fake size of the emulated flash",	0},
-	{"debug-type",		't',	"TYPE",		OPTION_HIDDEN,
+	{"debug-type",		't',	"N",		OPTION_HIDDEN,
 		"Set the type of the fake flash",	0},
 	{"debug-unit-test",	'u',	NULL,		OPTION_HIDDEN,
 		"Run a unit test; it ignores all other debug options",	0},
@@ -90,18 +90,16 @@ static error_t parse_opt(int key, char *arg, struct argp_state *state)
 		args->debug = true;
 		break;
 
-	case 't':
-		if (!strcmp(arg, "good"))
-			args->fake_type = FKTY_GOOD;
-		else if (!strcmp(arg, "limbo"))
-			args->fake_type = FKTY_LIMBO;
-		else if (!strcmp(arg, "wraparound"))
-			args->fake_type = FKTY_WRAPAROUND;
-		else
+	case 't': {
+		long l = arg_to_long(state, arg);
+		if (l < FKTY_GOOD || l >= FKTY_MAX)
 			argp_error(state,
-				"Fake type must be either `good', `limbo' or `wraparound'");
+				"Fake type must be a number in the interval [%i, %i]",
+				FKTY_GOOD, FKTY_MAX);
+		args->fake_type = l;
 		args->debug = true;
 		break;
+	}
 
 	case 'u':
 		args->unit_test = true;
@@ -140,6 +138,7 @@ struct unit_test_item {
 
 static const struct unit_test_item ftype_to_params[] = {
 	{FKTY_GOOD,		1, 1},
+	{FKTY_BAD,		0, 1},
 	{FKTY_LIMBO,		1, 2},
 	{FKTY_WRAPAROUND,	1, 2},
 	{FKTY_WRAPAROUND,	2, 3},
diff --git a/libprobe.c b/libprobe.c
index 6cf1abd..705a5ca 100644
--- a/libprobe.c
+++ b/libprobe.c
@@ -70,6 +70,10 @@ static int fdev_read_block(struct device *dev, char *buf, uint64_t block)
 	int done;
 
 	switch (fdev->fake_type) {
+	case FKTY_BAD:
+		memset(buf, 0, BLOCK_SIZE);
+		return 0;
+
 	case FKTY_LIMBO:
 		if (offset >= GIGABYTE * fdev->file_size_gb) {
 			memset(buf, 0, BLOCK_SIZE);
@@ -77,19 +81,11 @@ static int fdev_read_block(struct device *dev, char *buf, uint64_t block)
 		}
 		break;
 
-	/* XXX Support FKTY_TRUNCATE.
-	 * That is, it drops the highest bits, and addresses the real memory
-	 * with the resulting address.
-	 *
-	 * If @fake_size_gb % @file_size_gb == 0, it's identical to
-	 * FKTY_WRAPAROUND.
-	 */
-
 	case FKTY_WRAPAROUND:
 		offset %= GIGABYTE * fdev->file_size_gb;
 		/* Fall through. */
 
-	case  FKTY_GOOD:
+	case FKTY_GOOD:
 		break;
 
 	default:
@@ -131,6 +127,9 @@ static int fdev_write_block(struct device *dev, char *buf, uint64_t block)
 	off_t offset = block * BLOCK_SIZE;
 
 	switch (fdev->fake_type) {
+	case FKTY_BAD:
+		return 0;
+
 	case FKTY_LIMBO:
 		if (offset >= GIGABYTE * fdev->file_size_gb)
 			return 0;