diff --git a/lisp/compound-interest-test.lisp b/lisp/compound-interest-test.lisp
index 0f92231..47f1528 100644
--- a/lisp/compound-interest-test.lisp
+++ b/lisp/compound-interest-test.lisp
@@ -44,6 +44,9 @@
       (assert= 81450 (compound-interest 100000 -5 4))
     )
 
+    (defun negative-number-of-years ()
+      (assert= 100000 (compound-interest 100000 5 -4))
+    )
+
   )
 )
-
diff --git a/lisp/compound-interest.lisp b/lisp/compound-interest.lisp
index 263580f..c48f4de 100644
--- a/lisp/compound-interest.lisp
+++ b/lisp/compound-interest.lisp
@@ -1,20 +1,19 @@
 (defun decrement (n) (- n 1))
 
-(defun percent (n percent)
-  (if (> percent 0)
-    (/ (+ (* n percent) 50) 100)
-    (/ (- (* n percent) 50) 100)
+(defun percent (n percentage)
+  (if (> percentage 0)
+    (/ (+ (* n percentage) 50) 100)
+    (/ (- (* n percentage) 50) 100)
   )
 )
 
-(defun compound-interest (principal rate years)
-  (if (= years 0)
+(defun compound-interest (principal interest-rate years)
+  (if (< years 1)
     principal
     (compound-interest
-      (+ principal (percent principal rate))
-      rate
+      (+ principal (percent principal interest-rate))
+      interest-rate
       (decrement years)
     )
   )
 )
-