diff --git a/src/syntax/validate.rs b/src/syntax/validate.rs
index 676ff88..1c933da 100644
--- a/src/syntax/validate.rs
+++ b/src/syntax/validate.rs
@@ -1,6 +1,8 @@
-use crate::syntax::{Expression, Location, Program, Statement};
+use crate::{syntax::{Expression, Location, Program, Statement}, eval::PrimitiveType};
 use codespan_reporting::diagnostic::Diagnostic;
-use std::collections::HashMap;
+use std::{collections::HashMap, str::FromStr};
+
+use super::location;
 
 /// An error we found while validating the input program.
 ///
@@ -11,6 +13,7 @@ use std::collections::HashMap;
 /// and using [`codespan_reporting`] to present them to the user.
 pub enum Error {
     UnboundVariable(Location, String),
+    UnknownType(Location, String),
 }
 
 impl From<Error> for Diagnostic<usize> {
@@ -19,6 +22,10 @@ impl From<Error> for Diagnostic<usize> {
             Error::UnboundVariable(location, name) => location
                 .labelled_error("unbound here")
                 .with_message(format!("Unbound variable '{}'", name)),
+
+            Error::UnknownType(location, name) => location
+                .labelled_error("type referenced here")
+                .with_message(format!("Unknown type '{}'", name)),
         }
     }
 }
@@ -127,7 +134,15 @@ impl Expression {
                 vec![Error::UnboundVariable(loc.clone(), var.clone())],
                 vec![],
             ),
-            Expression::Cast(_, _, expr) => expr.validate(variable_map),
+            Expression::Cast(location, t, expr) => {
+                let (mut errs, warns) = expr.validate(variable_map);
+                
+                if PrimitiveType::from_str(t).is_err() {
+                    errs.push(Error::UnknownType(location.clone(), t.clone()))
+                }
+
+                (errs, warns)
+            }
             Expression::Primitive(_, _, args) => {
                 let mut errors = vec![];
                 let mut warnings = vec![];