diff --git a/alert.go b/alert.go
index 577cddc..a196135 100644
--- a/alert.go
+++ b/alert.go
@@ -12,6 +12,8 @@ import "strconv"
 // which wraps AlertError rather than sending a TLS alert.
 type AlertError uint8
 
+var _ error = AlertError(0)
+
 func (e AlertError) Error() string {
 	return alert(e).String()
 }
diff --git a/tls.go b/tls.go
index 970c99d..bdb0ed2 100644
--- a/tls.go
+++ b/tls.go
@@ -583,6 +583,8 @@ func Listen(network, laddr string, config *Config) (net.Listener, error) {
 
 type timeoutError struct{}
 
+var _ error = timeoutError{}
+
 func (timeoutError) Error() string   { return "tls: DialWithDialer timed out" }
 func (timeoutError) Timeout() bool   { return true }
 func (timeoutError) Temporary() bool { return true }