diff --git a/src/wasm-lib/kcl/src/parser/parser_impl.rs b/src/wasm-lib/kcl/src/parser/parser_impl.rs
index 6fc7d566c..21ebd220b 100644
--- a/src/wasm-lib/kcl/src/parser/parser_impl.rs
+++ b/src/wasm-lib/kcl/src/parser/parser_impl.rs
@@ -2801,4 +2801,5 @@ mod snapshot_tests {
     snapshot_test!(ar, r#"5 + "a""#);
     snapshot_test!(at, "line([0, l], %)");
     snapshot_test!(au, include_str!("../../../tests/executor/inputs/cylinder.kcl"));
+    snapshot_test!(av, "fn f = (angle) => { return default(angle, 360) }");
 }
diff --git a/src/wasm-lib/kcl/src/parser/snapshots/kcl_lib__parser__parser_impl__snapshot_tests__av.snap b/src/wasm-lib/kcl/src/parser/snapshots/kcl_lib__parser__parser_impl__snapshot_tests__av.snap
new file mode 100644
index 000000000..567f89421
--- /dev/null
+++ b/src/wasm-lib/kcl/src/parser/snapshots/kcl_lib__parser__parser_impl__snapshot_tests__av.snap
@@ -0,0 +1,94 @@
+---
+source: kcl/src/parser/parser_impl.rs
+expression: actual
+---
+{
+  "start": 0,
+  "end": 48,
+  "body": [
+    {
+      "type": "VariableDeclaration",
+      "type": "VariableDeclaration",
+      "start": 0,
+      "end": 48,
+      "declarations": [
+        {
+          "type": "VariableDeclarator",
+          "start": 3,
+          "end": 48,
+          "id": {
+            "type": "Identifier",
+            "start": 3,
+            "end": 4,
+            "name": "f"
+          },
+          "init": {
+            "type": "FunctionExpression",
+            "type": "FunctionExpression",
+            "start": 7,
+            "end": 48,
+            "params": [
+              {
+                "type": "Identifier",
+                "start": 8,
+                "end": 13,
+                "name": "angle"
+              }
+            ],
+            "body": {
+              "start": 18,
+              "end": 48,
+              "body": [
+                {
+                  "type": "ReturnStatement",
+                  "type": "ReturnStatement",
+                  "start": 20,
+                  "end": 46,
+                  "argument": {
+                    "type": "CallExpression",
+                    "type": "CallExpression",
+                    "start": 27,
+                    "end": 46,
+                    "callee": {
+                      "type": "Identifier",
+                      "start": 27,
+                      "end": 34,
+                      "name": "default"
+                    },
+                    "arguments": [
+                      {
+                        "type": "Identifier",
+                        "type": "Identifier",
+                        "start": 35,
+                        "end": 40,
+                        "name": "angle"
+                      },
+                      {
+                        "type": "Literal",
+                        "type": "Literal",
+                        "start": 42,
+                        "end": 45,
+                        "value": 360,
+                        "raw": "360"
+                      }
+                    ],
+                    "optional": false
+                  }
+                }
+              ],
+              "nonCodeMeta": {
+                "nonCodeNodes": {},
+                "start": []
+              }
+            }
+          }
+        }
+      ],
+      "kind": "fn"
+    }
+  ],
+  "nonCodeMeta": {
+    "nonCodeNodes": {},
+    "start": []
+  }
+}
diff --git a/src/wasm-lib/kcl/src/token.rs b/src/wasm-lib/kcl/src/token.rs
index 04a5a24ee..35367217e 100644
--- a/src/wasm-lib/kcl/src/token.rs
+++ b/src/wasm-lib/kcl/src/token.rs
@@ -47,6 +47,8 @@ pub enum TokenType {
     Function,
     /// Unknown lexemes.
     Unknown,
+    /// The ? symbol, used for optional values.
+    QuestionMark,
 }
 
 /// Most KCL tokens correspond to LSP semantic tokens (but not all).
@@ -58,6 +60,7 @@ impl TryFrom<TokenType> for SemanticTokenType {
             TokenType::Word => Self::VARIABLE,
             TokenType::Keyword => Self::KEYWORD,
             TokenType::Operator => Self::OPERATOR,
+            TokenType::QuestionMark => Self::OPERATOR,
             TokenType::String => Self::STRING,
             TokenType::LineComment => Self::COMMENT,
             TokenType::BlockComment => Self::COMMENT,
diff --git a/src/wasm-lib/kcl/src/token/tokeniser.rs b/src/wasm-lib/kcl/src/token/tokeniser.rs
index c56d5c226..f44d31bc8 100644
--- a/src/wasm-lib/kcl/src/token/tokeniser.rs
+++ b/src/wasm-lib/kcl/src/token/tokeniser.rs
@@ -21,6 +21,7 @@ pub fn token(i: &mut Located<&str>) -> PResult<Token> {
         '{' | '(' | '[' => brace_start,
         '}' | ')' | ']' => brace_end,
         ',' => comma,
+        '?' => question_mark,
         '0'..='9' => number,
         ':' => colon,
         '.' => alt((number, double_period, period)),
@@ -108,6 +109,11 @@ fn comma(i: &mut Located<&str>) -> PResult<Token> {
     Ok(Token::from_range(range, TokenType::Comma, value.to_string()))
 }
 
+fn question_mark(i: &mut Located<&str>) -> PResult<Token> {
+    let (value, range) = '?'.with_span().parse_next(i)?;
+    Ok(Token::from_range(range, TokenType::QuestionMark, value.to_string()))
+}
+
 fn colon(i: &mut Located<&str>) -> PResult<Token> {
     let (value, range) = ':'.with_span().parse_next(i)?;
     Ok(Token::from_range(range, TokenType::Colon, value.to_string()))