1 files changed, 4 insertions, 1 deletions
diff --git a/json.lisp b/json.lisp
index 6fe95b8..23c9198 100644
--- a/json.lisp
+++ b/json.lisp
@@ -12,7 +12,10 @@
 (defparameter number-literal
   (let ((signed-digits
           (comp ((sign (zero-or-one (some-char #\- #\+)))
-                 (natural (one-or-more (unit #'digit-char-p))))
+                 (natural (if sign
+                            (either (one-or-more (unit #'digit-char-p))
+                                    (fail "Malformed number."))
+                            (one-or-more (unit #'digit-char-p)))))
             (cons sign natural))))
     (comp ((base signed-digits)
            (dot (zero-or-one (the-char #\.)))