diff --git a/builtin.c b/builtin.c
index 3c3ac5bd..aa3c888a 100644
--- a/builtin.c
+++ b/builtin.c
@@ -287,6 +287,8 @@ static jv f_length(jv input) {
     return jv_number(jv_object_length(input));
   } else if (jv_get_kind(input) == JV_KIND_STRING) {
     return jv_number(jv_string_length_codepoints(input));
+  } else if (jv_get_kind(input) == JV_KIND_NUMBER) {
+    return jv_number(fabs(jv_number_value(input)));
   } else if (jv_get_kind(input) == JV_KIND_NULL) {
     jv_free(input);
     return jv_number(0);