diff --git a/setup.c b/setup.c
index 1563cd42df..c86bf5c9fa 100644
--- a/setup.c
+++ b/setup.c
@@ -102,7 +102,7 @@ char *prefix_path_gently(const char *prefix, int len,
 			return NULL;
 		}
 	} else {
-		sanitized = xstrfmt("%.*s%s", len, prefix, path);
+		sanitized = xstrfmt("%.*s%s", len, len ? prefix : "", path);
 		if (remaining_prefix)
 			*remaining_prefix = len;
 		if (normalize_path_copy_len(sanitized, sanitized, remaining_prefix)) {