diff --git a/acme/dns_challenge.go b/acme/dns_challenge.go
index bf837760..533431ca 100644
--- a/acme/dns_challenge.go
+++ b/acme/dns_challenge.go
@@ -18,7 +18,7 @@ type preCheckDNSFunc func(fqdn, value string) (bool, error)
 
 var (
 	preCheckDNS preCheckDNSFunc = checkDNSPropagation
-	fqdn2zone                   = map[string]string{}
+	fqdnToZone                  = map[string]string{}
 )
 
 var recursiveNameserver = "google-public-dns-a.google.com:53"
@@ -185,7 +185,7 @@ func lookupNameservers(fqdn string) ([]string, error) {
 // findZoneByFqdn determines the zone of the given fqdn
 func findZoneByFqdn(fqdn, nameserver string) (string, error) {
 	// Do we have it cached?
-	if zone, ok := fqdn2zone[fqdn]; ok {
+	if zone, ok := fqdnToZone[fqdn]; ok {
 		return zone, nil
 	}
 
@@ -210,9 +210,9 @@ func findZoneByFqdn(fqdn, nameserver string) (string, error) {
 				// If we ended up on one of the TLDs, it means the domain did not exist.
 				publicsuffix, _ := publicsuffix.PublicSuffix(unFqdn(zone))
 				if publicsuffix == unFqdn(zone) {
-					return "", fmt.Errorf("Could not determine zone authoritively")
+					return "", fmt.Errorf("Could not determine zone authoritatively")
 				}
-				fqdn2zone[fqdn] = zone
+				fqdnToZone[fqdn] = zone
 				return zone, nil
 			}
 		}
@@ -225,9 +225,9 @@ func findZoneByFqdn(fqdn, nameserver string) (string, error) {
 			// If we ended up on one of the TLDs, it means the domain did not exist.
 			publicsuffix, _ := publicsuffix.PublicSuffix(unFqdn(zone))
 			if publicsuffix == unFqdn(zone) {
-				return "", fmt.Errorf("Could not determine zone authoritively")
+				return "", fmt.Errorf("Could not determine zone authoritatively")
 			}
-			fqdn2zone[fqdn] = zone
+			fqdnToZone[fqdn] = zone
 			return zone, nil
 		}
 	}
@@ -252,6 +252,11 @@ func unFqdn(name string) string {
 	return name
 }
 
+// clearFqdnCache clears the cache of fqdn to zone mappings. Primarily used in testing.
+func clearFqdnCache() {
+	fqdnToZone = map[string]string{}
+}
+
 // waitFor polls the given function 'f', once every 'interval' seconds, up to 'timeout' seconds.
 func waitFor(timeout, interval int, f func() (bool, error)) error {
 	var lastErr string
diff --git a/acme/dns_challenge_rfc2136_test.go b/acme/dns_challenge_rfc2136_test.go
index b5905556..1a0ed5cc 100644
--- a/acme/dns_challenge_rfc2136_test.go
+++ b/acme/dns_challenge_rfc2136_test.go
@@ -26,7 +26,7 @@ var (
 var reqChan = make(chan *dns.Msg, 10)
 
 func TestRFC2136CanaryLocalTestServer(t *testing.T) {
-	fqdn2zone = map[string]string{}
+	clearFqdnCache()
 	dns.HandleFunc("example.com.", serverHandlerHello)
 	defer dns.HandleRemove("example.com.")
 
@@ -50,7 +50,7 @@ func TestRFC2136CanaryLocalTestServer(t *testing.T) {
 }
 
 func TestRFC2136ServerSuccess(t *testing.T) {
-	fqdn2zone = map[string]string{}
+	clearFqdnCache()
 	dns.HandleFunc(rfc2136TestZone, serverHandlerReturnSuccess)
 	defer dns.HandleRemove(rfc2136TestZone)
 
@@ -70,7 +70,7 @@ func TestRFC2136ServerSuccess(t *testing.T) {
 }
 
 func TestRFC2136ServerError(t *testing.T) {
-	fqdn2zone = map[string]string{}
+	clearFqdnCache()
 	dns.HandleFunc(rfc2136TestZone, serverHandlerReturnErr)
 	defer dns.HandleRemove(rfc2136TestZone)
 
@@ -92,7 +92,7 @@ func TestRFC2136ServerError(t *testing.T) {
 }
 
 func TestRFC2136TsigClient(t *testing.T) {
-	fqdn2zone = map[string]string{}
+	clearFqdnCache()
 	dns.HandleFunc(rfc2136TestZone, serverHandlerReturnSuccess)
 	defer dns.HandleRemove(rfc2136TestZone)
 
@@ -112,7 +112,7 @@ func TestRFC2136TsigClient(t *testing.T) {
 }
 
 func TestRFC2136ValidUpdatePacket(t *testing.T) {
-	fqdn2zone = map[string]string{}
+	clearFqdnCache()
 	dns.HandleFunc(rfc2136TestZone, serverHandlerPassBackRequest)
 	defer dns.HandleRemove(rfc2136TestZone)
 
diff --git a/acme/dns_challenge_test.go b/acme/dns_challenge_test.go
index 84b805f4..54d5e095 100644
--- a/acme/dns_challenge_test.go
+++ b/acme/dns_challenge_test.go
@@ -35,11 +35,15 @@ var lookupNameserversTestsErr = []struct {
 }{
 	// invalid tld
 	{"_null.n0n0.",
-		"Could not determine zone authoritively",
+		"Could not determine zone authoritatively",
 	},
 	// invalid domain
 	{"_null.com.",
-		"Could not determine zone authoritively",
+		"Could not determine zone authoritatively",
+	},
+	// invalid domain
+	{"in-valid.co.uk.",
+		"Could not determine zone authoritatively",
 	},
 }