diff --git a/exercises/03-fibonacci/fibonacci.go b/exercises/03-fibonacci/fibonacci.go
index ede80f0..426092b 100644
--- a/exercises/03-fibonacci/fibonacci.go
+++ b/exercises/03-fibonacci/fibonacci.go
@@ -13,10 +13,10 @@ var usage string = "usage: " + os.Args[0] + " <number of fibs>"
 // a function that returns an int.
 // from cpython/Doc/build/html/library/functools.html
 // [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610]
-func fibonacci() func() int {
-	cur_fib := 0
-	nxt_fib := 0
-	return func() int {
+func fibonacci() func() uint64 {
+	cur_fib := uint64(0)
+	nxt_fib := cur_fib
+	return func() uint64 {
 		if nxt_fib == 0 {
 			nxt_fib = 1
 			return 0
@@ -24,6 +24,9 @@ func fibonacci() func() int {
 		tmp := cur_fib
 		cur_fib = nxt_fib
 		nxt_fib += tmp
+		if nxt_fib < cur_fib {
+			panic("overfull!!")
+		}
 		return cur_fib
 	}
 }