diff --git a/engine/baml-lib/baml/tests/validation_files/class/invalid_type_aliases.baml b/engine/baml-lib/baml/tests/validation_files/class/invalid_type_aliases.baml
index 0a13664ba..e1d584c98 100644
--- a/engine/baml-lib/baml/tests/validation_files/class/invalid_type_aliases.baml
+++ b/engine/baml-lib/baml/tests/validation_files/class/invalid_type_aliases.baml
@@ -11,6 +11,9 @@ typpe Two = float
 // Unknown identifier.
 type Three = i
 
+// Unknown identifier in union.
+type Four = int | string | b
+
 // error: Error validating: Unexpected keyword used in assignment: typpe
 //   -->  class/invalid_type_aliases.baml:9
 //    | 
@@ -29,3 +32,9 @@ type Three = i
 // 11 | // Unknown identifier.
 // 12 | type Three = i
 //    | 
+// error: Error validating: Type alias points to unknown identifier `b`
+//   -->  class/invalid_type_aliases.baml:15
+//    | 
+// 14 | // Unknown identifier in union.
+// 15 | type Four = int | string | b
+//    | 
diff --git a/engine/baml-lib/parser-database/src/types/mod.rs b/engine/baml-lib/parser-database/src/types/mod.rs
index 1978fd078..59f2e73e7 100644
--- a/engine/baml-lib/parser-database/src/types/mod.rs
+++ b/engine/baml-lib/parser-database/src/types/mod.rs
@@ -518,7 +518,11 @@ fn visit_type_alias<'db>(
                 };
 
                 let Some(top_id) = ctx.names.tops.get(&string_id) else {
-                    unreachable!("Alias name `{ident}` is not registered in the context");
+                    ctx.push_error(DatamodelError::new_validation_error(
+                        &format!("Type alias points to unknown identifier `{ident}`"),
+                        item.span().clone(),
+                    ));
+                    return;
                 };
 
                 // Add alias to the graph.
diff --git a/engine/baml-schema-wasm/tests/test_file_manager.rs b/engine/baml-schema-wasm/tests/test_file_manager.rs
index bdcb841c2..6069e6091 100644
--- a/engine/baml-schema-wasm/tests/test_file_manager.rs
+++ b/engine/baml-schema-wasm/tests/test_file_manager.rs
@@ -231,7 +231,7 @@ test Two {
     }
 
     #[wasm_bindgen_test]
-    fn test_type_alias() {
+    fn test_type_alias_pointing_to_unknown_identifier() {
         wasm_logger::init(wasm_logger::Config::new(log::Level::Info));
         let sample_baml_content = r##"
             type Foo = i
@@ -265,4 +265,76 @@ test Two {
         //     .unwrap()
         // );
     }
+
+    #[wasm_bindgen_test]
+    fn test_type_alias_pointing_to_union_with_unknown_identifier() {
+        wasm_logger::init(wasm_logger::Config::new(log::Level::Info));
+        let sample_baml_content = r##"
+            type Foo = int | i
+        "##;
+        let mut files = HashMap::new();
+        files.insert("error.baml".to_string(), sample_baml_content.to_string());
+        let files_js = to_value(&files).unwrap();
+        let project = WasmProject::new("baml_src", files_js)
+            .map_err(JsValue::from)
+            .unwrap();
+
+        let env_vars = [("OPENAI_API_KEY", "12345")]
+            .iter()
+            .cloned()
+            .collect::<HashMap<_, _>>();
+        let env_vars_js = to_value(&env_vars).unwrap();
+
+        let Err(js_error) = project.runtime(env_vars_js) else {
+            panic!("Expected error, got Ok");
+        };
+
+        assert!(js_error.is_object());
+
+        // TODO: Don't know how to build Object
+        // assert_eq!(
+        //     js_error,
+        //     serde_wasm_bindgen::to_value::<HashMap<String, Vec<String>>>(&HashMap::from_iter([(
+        //         "all_files".to_string(),
+        //         vec!["error.baml".to_string()]
+        //     )]))
+        //     .unwrap()
+        // );
+    }
+
+    #[wasm_bindgen_test]
+    fn test_type_alias_pointing_to_union_with_unknown_identifier_in_union() {
+        wasm_logger::init(wasm_logger::Config::new(log::Level::Info));
+        let sample_baml_content = r##"
+            type Four = int | string | b
+        "##;
+        let mut files = HashMap::new();
+        files.insert("error.baml".to_string(), sample_baml_content.to_string());
+        let files_js = to_value(&files).unwrap();
+        let project = WasmProject::new("baml_src", files_js)
+            .map_err(JsValue::from)
+            .unwrap();
+
+        let env_vars = [("OPENAI_API_KEY", "12345")]
+            .iter()
+            .cloned()
+            .collect::<HashMap<_, _>>();
+        let env_vars_js = to_value(&env_vars).unwrap();
+
+        let Err(js_error) = project.runtime(env_vars_js) else {
+            panic!("Expected error, got Ok");
+        };
+
+        assert!(js_error.is_object());
+
+        // TODO: Don't know how to build Object
+        // assert_eq!(
+        //     js_error,
+        //     serde_wasm_bindgen::to_value::<HashMap<String, Vec<String>>>(&HashMap::from_iter([(
+        //         "all_files".to_string(),
+        //         vec!["error.baml".to_string()]
+        //     )]))
+        //     .unwrap()
+        // );
+    }
 }