diff --git a/.yarn/install-state.gz b/.yarn/install-state.gz
index 206b81c..b7da4fe 100644
Binary files a/.yarn/install-state.gz and b/.yarn/install-state.gz differ
diff --git a/packages/arbor-json/src/Json.ts b/packages/arbor-json/src/Json.ts
index 9736a27..004acb7 100644
--- a/packages/arbor-json/src/Json.ts
+++ b/packages/arbor-json/src/Json.ts
@@ -126,6 +126,8 @@ export class Json {
       } else if (Type) {
         const instance = new Type()
         return Object.assign(instance, value.$value)
+      } else {
+        throw new Error(`Unknown type: ${value.$type}. Can't deserialize.`)
       }
     }
 
diff --git a/packages/arbor-json/tests/Json.test.ts b/packages/arbor-json/tests/Json.test.ts
index d26728e..54c27ee 100644
--- a/packages/arbor-json/tests/Json.test.ts
+++ b/packages/arbor-json/tests/Json.test.ts
@@ -110,6 +110,15 @@ describe("Json", () => {
       expect(deserialized).toBeInstanceOf(TodoList)
       expect(deserialized).toEqual(todoList)
     })
+
+    it("throws exception when custom type is not found", () => {
+      const serialized =
+        '{"$value":{"uuid":"a","text":"Clean the house"},"$type":"Task"}'
+
+      expect(() => parse(serialized)).toThrow(
+        "Unknown type: Task. Can't deserialize."
+      )
+    })
   })
 
   describe("custom type with custom $type key", () => {