diff --git a/src/fib.ts b/src/fib.ts
index 8dc8ddd..bdc1b34 100644
--- a/src/fib.ts
+++ b/src/fib.ts
@@ -1,6 +1,6 @@
 // util function that computes the fibonacci numbers
-module.exports = function fibonacci(n) {
-  if (n < 0) {
+module.exports = function fibonacci(n): number {
+  if (!Number.isInteger(n) || n < 0) {
     return -1;
   } else if (n == 0) {
     return 0;