diff --git a/src/builtin/color/hsl.rs b/src/builtin/color/hsl.rs
index 5aa18a5..98f4c09 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/color/hsl.rs
@@ -107,9 +107,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n / Number::from(100),
+            v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.saturate(amount)))
     });
@@ -119,9 +118,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n / Number::from(100),
+           v => return Err(format!("$amount: {} is not a number.", v).into())
         };
         Ok(Value::Color(color.desaturate(amount)))
     });