diff --git a/fs/fs.c b/fs/fs.c
index 8fe2403..aaa6732 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -64,6 +64,15 @@ static inline void fs_close_unsupported(void)
 
 struct fstype_info {
 	int fstype;
+	/*
+	 * Is it legal to pass NULL as .probe()'s  fs_dev_desc parameter? This
+	 * should be false in most cases. For "virtual" filesystems which
+	 * aren't based on a U-Boot block device (e.g. sandbox), this can be
+	 * set to true. This should also be true for the dumm entry at the end
+	 * of fstypes[], since that is essentially a "virtual" (non-existent)
+	 * filesystem.
+	 */
+	bool null_dev_desc_ok;
 	int (*probe)(block_dev_desc_t *fs_dev_desc,
 		     disk_partition_t *fs_partition);
 	int (*ls)(const char *dirname);
@@ -77,6 +86,7 @@ static struct fstype_info fstypes[] = {
 #ifdef CONFIG_FS_FAT
 	{
 		.fstype = FS_TYPE_FAT,
+		.null_dev_desc_ok = false,
 		.probe = fat_set_blk_dev,
 		.close = fat_close,
 		.ls = file_fat_ls,
@@ -88,6 +98,7 @@ static struct fstype_info fstypes[] = {
 #ifdef CONFIG_FS_EXT4
 	{
 		.fstype = FS_TYPE_EXT,
+		.null_dev_desc_ok = false,
 		.probe = ext4fs_probe,
 		.close = ext4fs_close,
 		.ls = ext4fs_ls,
@@ -99,6 +110,7 @@ static struct fstype_info fstypes[] = {
 #ifdef CONFIG_SANDBOX
 	{
 		.fstype = FS_TYPE_SANDBOX,
+		.null_dev_desc_ok = true,
 		.probe = sandbox_fs_set_blk_dev,
 		.close = sandbox_fs_close,
 		.ls = sandbox_fs_ls,
@@ -109,6 +121,7 @@ static struct fstype_info fstypes[] = {
 #endif
 	{
 		.fstype = FS_TYPE_ANY,
+		.null_dev_desc_ok = true,
 		.probe = fs_probe_unsupported,
 		.close = fs_close_unsupported,
 		.ls = fs_ls_unsupported,
@@ -162,6 +175,9 @@ int fs_set_blk_dev(const char *ifname, const char *dev_part_str, int fstype)
 				fstype != info->fstype)
 			continue;
 
+		if (!fs_dev_desc && !info->null_dev_desc_ok)
+			continue;
+
 		if (!info->probe(fs_dev_desc, &fs_partition)) {
 			fs_type = info->fstype;
 			return 0;