diff --git a/src/builtin/functions/meta.rs b/src/builtin/functions/meta.rs
index b3c569c..71587d1 100644
--- a/src/builtin/functions/meta.rs
+++ b/src/builtin/functions/meta.rs
@@ -7,6 +7,7 @@ use crate::{
     common::{Identifier, QuoteKind},
     error::SassResult,
     parse::Parser,
+    unit::Unit,
     value::{SassFunction, Value},
 };
 
@@ -72,7 +73,17 @@ pub(crate) fn type_of(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult
 
 pub(crate) fn unitless(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
     args.max_args(1)?;
-    Ok(Value::bool(args.get_err(0, "number")?.unitless()))
+    Ok(match args.get_err(0, "number")? {
+        Value::Dimension(_, Unit::None, _) => Value::True,
+        Value::Dimension(..) => Value::False,
+        v => {
+            return Err((
+                format!("$number: {} is not a number.", v.inspect(args.span())?),
+                args.span(),
+            )
+                .into())
+        }
+    })
 }
 
 pub(crate) fn inspect(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult<Value> {
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 7e7291a..141c74e 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -380,15 +380,6 @@ impl Value {
         })
     }
 
-    pub fn unitless(&self) -> bool {
-        #[allow(clippy::match_same_arms)]
-        match self {
-            Value::Dimension(_, Unit::None, _) => true,
-            Value::Dimension(..) => false,
-            _ => true,
-        }
-    }
-
     pub fn not_equals(&self, other: &Self) -> bool {
         match self {
             Value::String(s1, ..) => match other {
diff --git a/tests/meta.rs b/tests/meta.rs
index c3c31ed..8bcf4ea 100644
--- a/tests/meta.rs
+++ b/tests/meta.rs
@@ -213,10 +213,9 @@ test!(
     "a {\n  color: unitless(1)\n}\n",
     "a {\n  color: true;\n}\n"
 );
-test!(
+error!(
     unitless_string,
-    "a {\n  color: unitless(foo)\n}\n",
-    "a {\n  color: true;\n}\n"
+    "a {\n  color: unitless(foo)\n}\n", "Error: $number: foo is not a number."
 );
 test!(
     variable_does_exist,