change to error and add regression test

src/operations/SnapshotEditor.ts

@@ -225,7 +225,8 @@ export class SnapshotEditor {
         }
 
         if (this._context.addTypename && payloadName === '__typename') {
-          warnings.push(`Encountered undefined payload value for __typename which will override __typename value on existing fragment.`);
+          const message = `Encountered undefined payload value for __typename which will override __typename value on existing fragment`;
+          throw new InvalidPayloadError(message, prefixPath, containerId, path, payload);
         }
       }
 

test/unit/Apollo/writeFragment/errorMissingTypename.ts

@@ -0,0 +1,37 @@
+import gql from 'graphql-tag';
+
+import { Hermes } from '../../../../src/apollo/Hermes';
+import { strictConfig } from '../../../helpers/context';
+import { StaticNodeId } from '../../../../src/schema';
+
+const { QueryRoot: QueryRootId } = StaticNodeId;
+
+describe(`writeFragment with missing __typename`, () => {
+
+  let hermes: Hermes;
+  beforeAll(() => {
+    hermes = new Hermes({
+      ...strictConfig,
+      addTypename: true,
+    });
+  });
+
+  it(`throws an error`, () => {
+    expect(() => {
+      hermes.writeFragment({
+        id: QueryRootId,
+        fragment: gql(`
+          fragment viewer on Viewer {
+            id
+            name
+          }
+        `),
+        data: {
+          id: 123,
+          name: 'Gouda',
+        },
+      });
+    }).to.throw(/InvalidPayloadError: Encountered undefined payload value for __typename/i);
+  });
+
+});