diff --git a/src/parse/media.rs b/src/parse/media.rs
index ac173d1..cbe2092 100644
--- a/src/parse/media.rs
+++ b/src/parse/media.rs
@@ -7,7 +7,7 @@ use crate::{
 use super::Parser;
 
 impl<'a> Parser<'a> {
-    pub fn scan_identifier(&mut self, ident: &str) -> SassResult<bool> {
+    pub fn scan_identifier(&mut self, ident: &'static str) -> SassResult<bool> {
         let peeked_identifier =
             match peek_ident_no_interpolation(self.toks, false, self.span_before) {
                 Ok(v) => v.node,
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 36d50d1..fe12241 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -17,10 +17,7 @@ use crate::{
         ComplexSelectorComponent, ExtendRule, ExtendedSelector, Extender, Selector, SelectorParser,
     },
     style::Style,
-    utils::{
-        peek_ident_no_interpolation, read_until_closing_curly_brace,
-        read_until_semicolon_or_closing_curly_brace,
-    },
+    utils::{read_until_closing_curly_brace, read_until_semicolon_or_closing_curly_brace},
     value::Value,
     Options, {Cow, Token},
 };
@@ -134,6 +131,16 @@ impl<'a> Parser<'a> {
         false
     }
 
+    pub fn expect_identifier(&mut self, ident: &'static str) -> SassResult<()> {
+        let this_ident = self.parse_identifier_no_interpolation(false)?;
+        self.span_before = this_ident.span;
+        if this_ident.node == ident {
+            return Ok(());
+        }
+
+        Err((format!("Expected \"{}\".", ident), self.span_before).into())
+    }
+
     fn parse_stmt(&mut self) -> SassResult<Vec<Stmt>> {
         let mut stmts = Vec::new();
         while let Some(Token { kind, pos }) = self.toks.peek() {
@@ -420,13 +427,11 @@ impl<'a> Parser<'a> {
                     }
                 }
                 '!' => {
-                    if peek_ident_no_interpolation(self.toks, false, self.span_before)?.node
-                        == "optional"
-                    {
-                        self.toks.truncate_iterator_to_cursor();
+                    if from_fn {
+                        self.expect_identifier("optional")?;
                         optional = true;
                     } else {
-                        string.push('!');
+                        return Err(("expected \"{\".", tok.pos).into());
                     }
                 }
                 c => string.push(c),
diff --git a/tests/extend.rs b/tests/extend.rs
index 47d0ab1..2e162d0 100644
--- a/tests/extend.rs
+++ b/tests/extend.rs
@@ -1882,6 +1882,13 @@ test!(
     }",
     ":has(a >) b, :has(a >) :has(a >) :has(a >) b, :has(a >) :has(a >) :has(a >) b {\n  color: red;\n}\n"
 );
+error!(
+    extend_optional_keyword_not_complete,
+    "a {
+        @extend a !opt;
+    }",
+    "Error: Expected \"optional\"."
+);
 
 // todo: extend_loop (massive test)
 // todo: extend tests in folders
diff --git a/tests/selectors.rs b/tests/selectors.rs
index 64fb23d..e4a4dc5 100644
--- a/tests/selectors.rs
+++ b/tests/selectors.rs
@@ -712,3 +712,7 @@ error!(
     toplevel_parent_selector_after_element,
     "a&{}", "Error: \"&\" may only used at the beginning of a compound selector."
 );
+error!(
+    denies_optional_in_selector,
+    "a !optional {}", "Error: expected \"{\"."
+);