diff --git a/TODO.md b/TODO.md
index 72061f7..89183b5 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,7 +1,5 @@
 # TODO
 
-- handle cert names for wildcard domains
-
 ## update readme
 
 - wrappers for ListenAndServeTLS
diff --git a/config.go b/config.go
index e1f9ffa..aaa1789 100644
--- a/config.go
+++ b/config.go
@@ -10,11 +10,19 @@ package simplecert
 
 import (
 	"errors"
+	"log"
 	"os"
 	"time"
 )
 
-var c *Config
+var (
+	c *Config
+
+	errNoDirectoryURL = errors.New("simplecert: no directory url specified")
+	errNoMail         = errors.New("simplecert: no SSLEmail in config")
+	errNoDomains      = errors.New("simplecert: no domains specified")
+	errNoChallenge    = errors.New("simplecert: no challenge method specified")
+)
 
 // Default contains a default configuration
 var Default = &Config{
@@ -82,16 +90,29 @@ type Config struct {
 
 // CheckConfig checks if config can be used to obtain a cert
 func CheckConfig(c *Config) error {
+
 	if len(c.Domains) == 0 {
-		return errors.New("simplecert: no domains specified")
+		return errNoDomains
 	}
 	if !c.Local {
 		if c.SSLEmail == "" {
-			return errors.New("simplecert: no SSLEmail in config")
+			return errNoMail
 		}
 	}
 	if c.DirectoryURL == "" {
-		return errors.New("simplecert: no directory url specified")
+		return errNoDirectoryURL
 	}
+
+	if c.DNSProvider == "" && c.HTTPAddress == "" && c.TLSAddress == "" {
+		return errNoChallenge
+	}
+
+	if c.WillRenewCertificate == nil && (c.HTTPAddress != "" || c.TLSAddress != "") {
+		log.Println("[WARNING] no WillRenewCertificate handler specified to handle graceful server shutdown")
+	}
+	if c.DidRenewCertificate == nil && (c.HTTPAddress != "" || c.TLSAddress != "") {
+		log.Println("[WARNING] no DidRenewCertificate handler specified to bring the service back up after renewing the certificate")
+	}
+
 	return nil
 }
diff --git a/utils.go b/utils.go
index 2f3db4c..eb41ba4 100644
--- a/utils.go
+++ b/utils.go
@@ -236,13 +236,17 @@ func renewalRoutine(cr *certificate.Resource) {
 		time.Sleep(c.CheckInterval)
 
 		// allow graceful shutdown of running services if required
-		c.WillRenewCertificate()
+		if c.WillRenewCertificate != nil {
+			c.WillRenewCertificate()
+		}
 
 		// renew the certificate
 		renew(cr)
 
 		// allow service restart if required
-		c.DidRenewCertificate()
+		if c.DidRenewCertificate != nil {
+			c.DidRenewCertificate()
+		}
 	}
 }