diff --git a/devices/devices.go b/devices/devices.go
index 7a61e36..e27a953 100644
--- a/devices/devices.go
+++ b/devices/devices.go
@@ -48,7 +48,11 @@ func (m Milli) Float64() float64 {
 
 // String returns the value formatted as a string.
 func (m Milli) String() string {
-	return fmt.Sprintf("%d.%03d", m/1000, m%1000)
+	d := m % 1000
+	if d < 0 {
+		d = -d
+	}
+	return fmt.Sprintf("%d.%03d", m/1000, d)
 }
 
 // Celsius is a temperature at a precision of 0.001°C.
diff --git a/devices/devices_test.go b/devices/devices_test.go
index b60cbf5..6b095e1 100644
--- a/devices/devices_test.go
+++ b/devices/devices_test.go
@@ -16,6 +16,16 @@ func TestMilli(t *testing.T) {
 	}
 }
 
+func TestMilli_neg(t *testing.T) {
+	o := Milli(-10010)
+	if s := o.String(); s != "-10.010" {
+		t.Fatalf("%#v", s)
+	}
+	if f := o.Float64(); f > -10.009 || f < -10.011 {
+		t.Fatalf("%f", f)
+	}
+}
+
 func TestCelsius(t *testing.T) {
 	o := Celsius(10010)
 	if s := o.String(); s != "10.010°C" {