diff --git a/bbb.go b/bbb.go
index bf0aef6..db4023a 100644
--- a/bbb.go
+++ b/bbb.go
@@ -16,7 +16,7 @@ import (
 )
 
 func init() {
-	Describers[HostBBB] = func(rev int) *Descriptor {
+	Register(HostBBB, func(rev int) *Descriptor {
 		return &Descriptor{
 			GPIODriver: func() GPIODriver {
 				return newGPIODriver(bbbPins, newDigitalPin, newBBBAnalogPin)
@@ -26,7 +26,7 @@ func init() {
 				return newLEDDriver(bbbLEDMap)
 			},
 		}
-	}
+	})
 }
 
 var bbbPins = PinMap{
diff --git a/descriptor.go b/descriptor.go
index 2d161b9..275edaa 100644
--- a/descriptor.go
+++ b/descriptor.go
@@ -18,7 +18,17 @@ type Descriptor struct {
 type Describer func(rev int) *Descriptor
 
 // Describers is a global list of registered host Describers.
-var Describers = map[Host]Describer{}
+var describers = make(map[Host]Describer)
+
+func Register(host Host, describer Describer) {
+	if describer == nil {
+		panic("embd: describer is nil")
+	}
+	if _, dup := describers[host]; dup {
+		panic("embd: describer already registered")
+	}
+	describers[host] = describer
+}
 
 // DescribeHost returns the detected host descriptor.
 func DescribeHost() (*Descriptor, error) {
@@ -27,7 +37,7 @@ func DescribeHost() (*Descriptor, error) {
 		return nil, err
 	}
 
-	describer, ok := Describers[host]
+	describer, ok := describers[host]
 	if !ok {
 		return nil, fmt.Errorf("host: invalid host %q", host)
 	}
diff --git a/rpi.go b/rpi.go
index 90ca61e..a5abc32 100644
--- a/rpi.go
+++ b/rpi.go
@@ -7,7 +7,7 @@
 package embd
 
 func init() {
-	Describers[HostRPi] = func(rev int) *Descriptor {
+	Register(HostRPi, func(rev int) *Descriptor {
 		var pins = rpiRev1Pins
 		if rev > 1 {
 			pins = rpiRev2Pins
@@ -19,7 +19,7 @@ func init() {
 			},
 			I2CDriver: newI2CDriver,
 		}
-	}
+	})
 }
 
 var rpiRev1Pins = PinMap{