diff --git a/src/parse/value/eval.rs b/src/parse/value/eval.rs
index aa46722..02c3f19 100644
--- a/src/parse/value/eval.rs
+++ b/src/parse/value/eval.rs
@@ -317,9 +317,9 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
                 format!("-{}", right.to_css_string(self.span)?),
                 QuoteKind::None,
             ),
-            Value::Dimension(None, ..) => todo!(),
+            v @ Value::Dimension(None, ..) => v,
             Value::Dimension(Some(num), unit, _) => match right {
-                Value::Dimension(None, ..) => todo!(),
+                v @ Value::Dimension(None, ..) => v,
                 Value::Dimension(Some(num2), unit2, _) => {
                     if !unit.comparable(&unit2) {
                         return Err((
@@ -643,9 +643,9 @@ impl<'a, 'b: 'a> ValueVisitor<'a, 'b> {
             v => panic!("{:?}", v),
         };
         Ok(match left {
-            Value::Dimension(None, ..) => todo!(),
+            v @ Value::Dimension(None, ..) => v,
             Value::Dimension(Some(n), u, _) => match right {
-                Value::Dimension(None, ..) => todo!(),
+                v @ Value::Dimension(None, ..) => v,
                 Value::Dimension(Some(n2), u2, _) => {
                     if !u.comparable(&u2) {
                         return Err(
diff --git a/tests/subtraction.rs b/tests/subtraction.rs
index f883fe8..ea507fd 100644
--- a/tests/subtraction.rs
+++ b/tests/subtraction.rs
@@ -282,6 +282,16 @@ test!(
     "a {\n  color: foo - red;\n}\n",
     "a {\n  color: foo-red;\n}\n"
 );
+test!(
+    sub_nan_left,
+    "a {\n  left:0/0-0;\n}\n",
+    "a {\n  left: NaN;\n}\n"
+);
+test!(
+    sub_nan_right,
+    "a {\n  left:0-0/0;\n}\n",
+    "a {\n  left: NaN;\n}\n"
+);
 error!(
     number_minus_color,
     "a {\n  color: 1 - #abc;\n}\n", "Error: Undefined operation \"1 - #abc\"."