diff --git a/plugins/primus_lisp/lisp/ascii.lisp b/plugins/primus_lisp/lisp/ascii.lisp
index 060a48c25..35941696f 100644
--- a/plugins/primus_lisp/lisp/ascii.lisp
+++ b/plugins/primus_lisp/lisp/ascii.lisp
@@ -10,8 +10,11 @@
       (= s 32)))
 
 (defun ascii-sign (s)
-  "(ascii-sign S) is 1 if S is + and -1 otherwise"
-  (if (= s ?+) 1 -1))
+  "(ascii-sign S) is 1 if S is +, -1 if it -, or 0 otherwise"
+  (case s
+    ?- -1
+    ?+  1
+    0))
 
 (defun ascii-is-digit (s)
   "(ascii-is-digit s) is true if S is an ascii representation of decimal digit"
diff --git a/plugins/primus_lisp/lisp/atoi.lisp b/plugins/primus_lisp/lisp/atoi.lisp
index 9e6513f96..ec1044bea 100644
--- a/plugins/primus_lisp/lisp/atoi.lisp
+++ b/plugins/primus_lisp/lisp/atoi.lisp
@@ -14,6 +14,9 @@
   (skip-all atoi-prefix s)
   (let ((v 0)
         (sign (ascii-sign (memory-read s))))
+    (if (= sign 0)
+        (set sign 1)
+      (incr s))
     (while (ascii-is-digit (memory-read s))
       (set v (+ (* v 10) (atoi-read-digit s)))
       (incr s))