diff --git a/leading_zeros_18.go b/leading_zeros_18.go
index cd10a88..72af1d6 100644
--- a/leading_zeros_18.go
+++ b/leading_zeros_18.go
@@ -21,7 +21,7 @@ var len8tab = "" +
 	"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08" +
 	"\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08"
 
-// Len64 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
+// len64 returns the minimum number of bits required to represent x; the result is 0 for x == 0.
 func len64(x uint64) (n uint) {
 	if x >= 1<<32 {
 		x >>= 32
@@ -39,5 +39,5 @@ func len64(x uint64) (n uint) {
 }
 
 func leadingZeroes64(v uint64) uint {
-	return 64 - len64(x)
+	return 64 - len64(v)
 }
diff --git a/leading_zeros_test.go b/leading_zeros_test.go
new file mode 100644
index 0000000..7c2b1ec
--- /dev/null
+++ b/leading_zeros_test.go
@@ -0,0 +1,18 @@
+//go:build !go1.9
+// +build !go1.9
+
+package bitset
+
+import (
+	"testing"
+)
+
+func TestLen64(t *testing.T) {
+	for i := 0; i < 64; i++ {
+		received := len64(uint64(1) << i)
+		expected := uint(i + 1)
+		if received != expected {
+			t.Errorf("len64(%b) is incorrect: received %d, expected %d", uint64(1)<<i, received, expected)
+		}
+	}
+}