diff --git a/devices/devices.go b/devices/devices.go
index f7c6f84..63d2378 100644
--- a/devices/devices.go
+++ b/devices/devices.go
@@ -129,7 +129,11 @@ func (r RelativeHumidity) Float64() float64 {
 
 // String returns the humidity formatted as a string.
 func (r RelativeHumidity) String() string {
-	return fmt.Sprintf("%d.%02d%%rH", r/100, r%100)
+	m := r % 100
+	if m < 0 {
+		m = -m
+	}
+	return fmt.Sprintf("%d.%02d%%rH", r/100, m)
 }
 
 // Environment represents measurements from an environmental sensor.
diff --git a/devices/devices_test.go b/devices/devices_test.go
index 6b095e1..180d990 100644
--- a/devices/devices_test.go
+++ b/devices/devices_test.go
@@ -68,3 +68,13 @@ func TestRelativeHumidity(t *testing.T) {
 		t.Fatalf("%f", f)
 	}
 }
+
+func TestRelativeHumidity_neg(t *testing.T) {
+	o := RelativeHumidity(-5010)
+	if s := o.String(); s != "-50.10%rH" {
+		t.Fatalf("%#v", s)
+	}
+	if f := o.Float64(); f < -50.11 || f > -50.09 {
+		t.Fatalf("%f", f)
+	}
+}