diff --git a/src/value/mod.rs b/src/value/mod.rs
index f24ed4f..716e71a 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -82,11 +82,11 @@ fn visit_quoted_string(buf: &mut String, force_double_quote: bool, string: &str)
                     buffer.push(hex_char_for(c as u32 >> 4))
                 }
                 buffer.push(hex_char_for(c as u32 & 0xF));
-                if iter.peek().is_none() {
-                    break;
-                }
 
-                let next = iter.peek().unwrap();
+                let next = match iter.peek() {
+                    Some(v) => v,
+                    None => break,
+                };
 
                 if next.is_ascii_hexdigit() || next == &' ' || next == &'\t' {
                     buffer.push(' ');
@@ -148,7 +148,7 @@ impl Value {
             Self::List(vals, sep, brackets) => match brackets {
                 Brackets::None => Cow::owned(
                     vals.iter()
-                        .filter(|x| !x.is_null(span).unwrap())
+                        .filter(|x| !x.clone().is_null(span).unwrap_or(false))
                         .map(|x| x.to_css_string(span))
                         .collect::<SassResult<Vec<Cow<'static, str>>>>()?
                         .join(sep.as_str()),
@@ -156,7 +156,7 @@ impl Value {
                 Brackets::Bracketed => Cow::owned(format!(
                     "[{}]",
                     vals.iter()
-                        .filter(|x| !x.is_null(span).unwrap())
+                        .filter(|x| !x.is_null(span).unwrap_or(false))
                         .map(|x| x.to_css_string(span))
                         .collect::<SassResult<Vec<Cow<'static, str>>>>()?
                         .join(sep.as_str()),
@@ -199,7 +199,7 @@ impl Value {
             Self::Null => Cow::const_str(""),
             Self::ArgList(args) => Cow::owned(
                 args.iter()
-                    .filter(|x| !x.is_null(span).unwrap())
+                    .filter(|x| !x.is_null(span).unwrap_or(false))
                     .map(|a| Ok(a.node.to_css_string(span)?))
                     .collect::<SassResult<Vec<Cow<'static, str>>>>()?
                     .join(", "),
diff --git a/tests/list.rs b/tests/list.rs
index e68c5ae..a396edd 100644
--- a/tests/list.rs
+++ b/tests/list.rs
@@ -314,3 +314,19 @@ test!(
     "a {\n  color: 3;\n}\n"
 );
 test!(empty_bracketed_list, "a {\n  empty: [];\n}\n");
+error!(
+    invalid_item_in_space_separated_list,
+    "a {\n  color: red color * #abc;\n}\n", "Error: Undefined operation \"color * #abc\"."
+);
+error!(
+    invalid_item_in_comma_separated_list,
+    "a {\n  color: red, color * #abc;\n}\n", "Error: Undefined operation \"color * #abc\"."
+);
+error!(
+    invalid_item_in_space_separated_list_inside_interpolation,
+    "a {\n  color: #{red color * #abc};\n}\n", "Error: Undefined operation \"color * #abc\"."
+);
+error!(
+    invalid_item_in_comma_separated_list_inside_interpolation,
+    "a {\n  color: #{red, color * #abc};\n}\n", "Error: Undefined operation \"color * #abc\"."
+);