diff --git a/CHANGELOG.md b/CHANGELOG.md
index cc6244d9387..123b4e76433 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,9 @@
 - Implement `useReactiveVar` hook for consuming reactive variables in React components. <br/>
   [@benjamn](https://github.com/benjamn) in [#6867](https://github.com/apollographql/apollo-client/pull/6867)
 
+- Throw if `writeFragment` cannot identify `options.data` when no `options.id` provided. <br/>
+  [@jcreighton](https://github.com/jcreighton) in [#6859](https://github.com/apollographql/apollo-client/pull/6859)
+
 ## Apollo Client 3.1.3
 
 ## Bug Fixes
diff --git a/src/cache/inmemory/__tests__/writeToStore.ts b/src/cache/inmemory/__tests__/writeToStore.ts
index 9e39a45b9b2..aae63df4703 100644
--- a/src/cache/inmemory/__tests__/writeToStore.ts
+++ b/src/cache/inmemory/__tests__/writeToStore.ts
@@ -2325,4 +2325,17 @@ describe('writing to the store', () => {
       },
     });
   });
+
+  it("should warn if it cannot identify the result object", () => {
+    const cache = new InMemoryCache;
+
+    expect(() => {
+      cache.writeFragment({
+        fragment: gql`fragment Count on Counter { count }`,
+        data: {
+          count: 1,
+        },
+      });
+    }).toThrowError(/Could not identify object/);
+  });
 });
diff --git a/src/cache/inmemory/writeToStore.ts b/src/cache/inmemory/writeToStore.ts
index fea1d64bf5c..25feb11be28 100644
--- a/src/cache/inmemory/writeToStore.ts
+++ b/src/cache/inmemory/writeToStore.ts
@@ -89,7 +89,7 @@ export class StoreWriter {
       ...variables,
     };
 
-    const objOrRef = this.processSelectionSet({
+    const ref = this.processSelectionSet({
       result: result || Object.create(null),
       dataId,
       selectionSet: operationDefinition.selectionSet,
@@ -105,15 +105,14 @@ export class StoreWriter {
       },
     });
 
-    const ref = isReference(objOrRef) ? objOrRef :
-      dataId && makeReference(dataId) || void 0;
-
-    if (ref) {
-      // Any IDs written explicitly to the cache (including ROOT_QUERY,
-      // most frequently) will be retained as reachable root IDs.
-      store.retain(ref.__ref);
+    if (!isReference(ref)) {
+      throw new InvariantError(`Could not identify object ${JSON.stringify(result)}`);
     }
 
+    // Any IDs written explicitly to the cache (including ROOT_QUERY,
+    // most frequently) will be retained as reachable root IDs.
+    store.retain(ref.__ref);
+
     return ref;
   }