diff --git a/src/error.rs b/src/error.rs
index 20057df..a0d2f88 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -10,6 +10,7 @@ pub enum Error {
     Parse { expected: String, span: SimpleSpan },
     Lex { expected: String, span: SimpleSpan },
     Runtime(RuntimeError),
+    Validation(ValidationError),
 }
 
 impl From<Rich<'_, char>> for Error {
@@ -92,7 +93,7 @@ pub fn create_report<'a>(errors: &'a [Error]) -> Report<'a> {
     let mut report = Report::build(ReportKind::Error, (), 0);
 
     for error in errors {
-        match error {
+        match &error {
             Error::Parse { expected, span } => {
                 report = report.with_label(
                     Label::new(span.start..span.end).with_message(format!("Expected {expected}.")),
@@ -109,6 +110,7 @@ pub fn create_report<'a>(errors: &'a [Error]) -> Report<'a> {
                 );
             }
             Error::Runtime(_) => todo!(),
+            Error::Validation(_) => todo!(),
         }
     }
 
diff --git a/src/lib.rs b/src/lib.rs
index ab09e0e..9282dc2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,6 +24,20 @@ impl Interpreter {
     pub fn run(&mut self, source: &str) -> Result<Value, Vec<Error>> {
         let tokens = lex(source)?;
         let statements = parse(&tokens)?;
+        let errors = statements
+            .iter()
+            .filter_map(|(statement, _span)| {
+                if let Some(error) = statement.validate(&self.context).err() {
+                    Some(Error::Validation(error))
+                } else {
+                    None
+                }
+            })
+            .collect::<Vec<Error>>();
+
+        if !errors.is_empty() {
+            return Err(errors);
+        }
 
         let mut value = Value::none();