return null when data from an id cannot be found

src/ApolloClient.ts

@@ -385,13 +385,23 @@ export default class ApolloClient {
     fragment: DocumentNode,
     fragmentName?: string,
     variables?: Object,
-  ): FragmentType {
+  ): FragmentType | null {
     this.initStore();
+
+    const query = getFragmentQuery(fragment, fragmentName);
     const reduxRootSelector = this.reduxRootSelector || defaultReduxRootSelector;
+    const store = reduxRootSelector(this.store.getState()).data;
+
+    // If we could not find an item in the store with the provided id then we
+    // just return `null`.
+    if (typeof store[id] === 'undefined') {
+      return null;
+    }
+
     return readQueryFromStore<FragmentType>({
       rootId: id,
-      store: reduxRootSelector(this.store.getState()).data,
-      query: getFragmentQuery(fragment, fragmentName),
+      store,
+      query,
       variables,
       returnPartialData: false,
     });

test/ApolloClient.ts

@@ -226,6 +226,32 @@ describe('ApolloClient', () => {
         },
       ), { a: 1, b: 2 });
     });
+
+    it('will return null when an id that can’t be found is provided', () => {
+      const client1 = new ApolloClient();
+      const client2 = new ApolloClient({
+        initialState: {
+          apollo: {
+            data: {
+              'bar': { __typename: 'Type1', a: 1, b: 2, c: 3 },
+            },
+          },
+        },
+      });
+      const client3 = new ApolloClient({
+        initialState: {
+          apollo: {
+            data: {
+              'foo': { __typename: 'Type1', a: 1, b: 2, c: 3 },
+            },
+          },
+        },
+      });
+
+      assert.equal(client1.readFragment('foo', gql`fragment fooFragment on Foo { a b c }`), null);
+      assert.equal(client2.readFragment('foo', gql`fragment fooFragment on Foo { a b c }`), null);
+      assert.deepEqual(client3.readFragment('foo', gql`fragment fooFragment on Foo { a b c }`), { a: 1, b: 2, c: 3 });
+    });
   });
 
   describe('writeQuery', () => {