diff --git a/src/cache/inmemory/__tests__/object-canon.ts b/src/cache/inmemory/__tests__/object-canon.ts
index 83014a7f3bb..8af77d614cf 100644
--- a/src/cache/inmemory/__tests__/object-canon.ts
+++ b/src/cache/inmemory/__tests__/object-canon.ts
@@ -46,7 +46,9 @@ describe("ObjectCanon", () => {
     expect(canon.admit(c2)).toBe(c2);
   });
 
-  it("preserves custom prototypes", () => {
+  // TODO Reenable this when ObjectCanon allows enabling canonization for
+  // arbitrary prototypes (not just {Array,Object}.prototype and null).
+  it.skip("preserves custom prototypes", () => {
     const canon = new ObjectCanon;
 
     class Custom {
diff --git a/src/cache/inmemory/__tests__/readFromStore.ts b/src/cache/inmemory/__tests__/readFromStore.ts
index 3df74615432..54d9e691741 100644
--- a/src/cache/inmemory/__tests__/readFromStore.ts
+++ b/src/cache/inmemory/__tests__/readFromStore.ts
@@ -2044,11 +2044,11 @@ describe('reading from the store', () => {
     }
 
     const nonCanonicalQueryResult0 = readQuery(false);
-    expect(canon.isCanonical(nonCanonicalQueryResult0)).toBe(false);
+    expect(canon.isKnown(nonCanonicalQueryResult0)).toBe(false);
     expect(nonCanonicalQueryResult0).toEqual({ count: 0 });
 
     const canonicalQueryResult0 = readQuery(true);
-    expect(canon.isCanonical(canonicalQueryResult0)).toBe(true);
+    expect(canon.isKnown(canonicalQueryResult0)).toBe(true);
     // The preservation of { count: 0 } proves the result didn't have to be
     // recomputed, but merely canonized.
     expect(canonicalQueryResult0).toEqual({ count: 0 });
@@ -2058,7 +2058,7 @@ describe('reading from the store', () => {
     });
 
     const canonicalQueryResult1 = readQuery(true);
-    expect(canon.isCanonical(canonicalQueryResult1)).toBe(true);
+    expect(canon.isKnown(canonicalQueryResult1)).toBe(true);
     expect(canonicalQueryResult1).toEqual({ count: 1 });
 
     const nonCanonicalQueryResult1 = readQuery(false);
@@ -2101,7 +2101,7 @@ describe('reading from the store', () => {
     }
 
     const canonicalFragmentResult1 = readFragment(true);
-    expect(canon.isCanonical(canonicalFragmentResult1)).toBe(true);
+    expect(canon.isKnown(canonicalFragmentResult1)).toBe(true);
     expect(canonicalFragmentResult1).toEqual({ count: 0 });
 
     const nonCanonicalFragmentResult1 = readFragment(false);
@@ -2115,13 +2115,13 @@ describe('reading from the store', () => {
 
     const nonCanonicalFragmentResult2 = readFragment(false);
     expect(readFragment(false)).toBe(nonCanonicalFragmentResult2);
-    expect(canon.isCanonical(nonCanonicalFragmentResult2)).toBe(false);
+    expect(canon.isKnown(nonCanonicalFragmentResult2)).toBe(false);
     expect(nonCanonicalFragmentResult2).toEqual({ count: 1 });
     expect(readFragment(false)).toBe(nonCanonicalFragmentResult2);
 
     const canonicalFragmentResult2 = readFragment(true);
     expect(readFragment(true)).toBe(canonicalFragmentResult2);
-    expect(canon.isCanonical(canonicalFragmentResult2)).toBe(true);
+    expect(canon.isKnown(canonicalFragmentResult2)).toBe(true);
     expect(canonicalFragmentResult2).toEqual({ count: 1 });
   });
 });
diff --git a/src/cache/inmemory/helpers.ts b/src/cache/inmemory/helpers.ts
index 9777c8330a0..b4627cab30b 100644
--- a/src/cache/inmemory/helpers.ts
+++ b/src/cache/inmemory/helpers.ts
@@ -14,7 +14,6 @@ import {
 
 export const {
   hasOwnProperty: hasOwn,
-  toString: objToStr,
 } = Object.prototype;
 
 export function getTypenameFromStoreObject(
diff --git a/src/cache/inmemory/object-canon.ts b/src/cache/inmemory/object-canon.ts
index d9a2747e2a7..7c76971d220 100644
--- a/src/cache/inmemory/object-canon.ts
+++ b/src/cache/inmemory/object-canon.ts
@@ -1,6 +1,5 @@
 import { Trie } from "@wry/trie";
 import { canUseWeakMap } from "../../utilities";
-import { objToStr } from "./helpers";
 
 function isObjectOrArray(value: any): value is object {
   return !!value && typeof value === "object";
@@ -82,7 +81,7 @@ export class ObjectCanon {
     keys?: SortedKeysInfo;
   }>(canUseWeakMap);
 
-  public isCanonical(value: any): boolean {
+  public isKnown(value: any): boolean {
     return isObjectOrArray(value) && this.known.has(value);
   }
 
@@ -106,8 +105,9 @@ export class ObjectCanon {
       const original = this.passes.get(value);
       if (original) return original;
 
-      switch (objToStr.call(value)) {
-        case "[object Array]": {
+      const proto = Object.getPrototypeOf(value);
+      switch (proto) {
+        case Array.prototype: {
           if (this.known.has(value)) return value;
           const array: any[] = (value as any[]).map(this.admit, this);
           // Arrays are looked up in the Trie using their recursively
@@ -126,7 +126,8 @@ export class ObjectCanon {
           return node.array;
         }
 
-        case "[object Object]": {
+        case null:
+        case Object.prototype: {
           if (this.known.has(value)) return value;
           const proto = Object.getPrototypeOf(value);
           const array = [proto];
diff --git a/src/cache/inmemory/readFromStore.ts b/src/cache/inmemory/readFromStore.ts
index 2572c6c0607..803855c7526 100644
--- a/src/cache/inmemory/readFromStore.ts
+++ b/src/cache/inmemory/readFromStore.ts
@@ -273,7 +273,7 @@ export class StoreReader {
         // If result is canonical, then it could only have been previously
         // cached by the canonizing version of executeSelectionSet, so we can
         // avoid checking both possibilities here.
-        this.canon.isCanonical(result),
+        this.canon.isKnown(result),
       );
       if (latest && result === latest.result) {
         return true;