diff --git a/utils.c b/utils.c
index acbdf9f2..16eefc62 100644
--- a/utils.c
+++ b/utils.c
@@ -2579,7 +2579,7 @@ out:
  * Note: this function uses a static per-thread buffer. Do not call this
  * function more than 10 times within one argument list!
  */
-const char *pretty_size_mode(u64 size, unsigned mode)
+const char *pretty_size_mode(s64 size, unsigned mode)
 {
 	static __thread int ps_index = 0;
 	static __thread char ps_array[10][32];
@@ -2598,20 +2598,20 @@ static const char* unit_suffix_binary[] =
 static const char* unit_suffix_decimal[] =
 	{ "B", "kB", "MB", "GB", "TB", "PB", "EB"};
 
-int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mode)
+int pretty_size_snprintf(s64 size, char *str, size_t str_size, unsigned unit_mode)
 {
 	int num_divs;
 	float fraction;
-	u64 base = 0;
+	s64 base = 0;
 	int mult = 0;
 	const char** suffix = NULL;
-	u64 last_size;
+	s64 last_size;
 
 	if (str_size == 0)
 		return 0;
 
 	if ((unit_mode & ~UNITS_MODE_MASK) == UNITS_RAW) {
-		snprintf(str, str_size, "%llu", size);
+		snprintf(str, str_size, "%lld", size);
 		return 0;
 	}
 
@@ -2646,7 +2646,7 @@ int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mod
 			   num_divs = 0;
 			   break;
 	default:
-		while (size >= mult) {
+		while ((size < 0 ? -size : size) >= mult) {
 			last_size = size;
 			size /= mult;
 			num_divs++;
diff --git a/utils.h b/utils.h
index 366ca292..525bde9f 100644
--- a/utils.h
+++ b/utils.h
@@ -174,9 +174,9 @@ int check_mounted_where(int fd, const char *file, char *where, int size,
 int btrfs_device_already_in_root(struct btrfs_root *root, int fd,
 				 int super_offset);
 
-int pretty_size_snprintf(u64 size, char *str, size_t str_bytes, unsigned unit_mode);
+int pretty_size_snprintf(s64 size, char *str, size_t str_bytes, unsigned unit_mode);
 #define pretty_size(size) 	pretty_size_mode(size, UNITS_DEFAULT)
-const char *pretty_size_mode(u64 size, unsigned mode);
+const char *pretty_size_mode(s64 size, unsigned mode);
 
 u64 parse_size(char *s);
 u64 parse_qgroupid(const char *p);