diff --git a/src/boot/replaceRevive.js b/src/boot/replaceRevive.js
index c9f43746500..7c1eb0e9f39 100644
--- a/src/boot/replaceRevive.js
+++ b/src/boot/replaceRevive.js
@@ -23,6 +23,13 @@ export const SERIALIZED_TYPE_FIELD_NAME: '__serializedType__' = '__serializedTyp
  */
 const SERIALIZED_TYPE_FIELD_NAME_ESCAPED: '__serializedType__value' = '__serializedType__value';
 
+/**
+ * Custom replacer for inventive data types JSON doesn't handle.
+ *
+ * To be passed to `JSON.stringify` as its second argument. New
+ * replacement logic must also appear in `reviver` so they stay in
+ * sync.
+ */
 // Don't make this an arrow function -- we need `this` to be a special
 // value; see
 // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#The_replacer_parameter.
@@ -30,65 +37,87 @@ const replacer = function replacer(key, value) {
   // The value at the current path before JSON.stringify called its
   // `toJSON` method, if present.
   //
-  // When identifying what kind of thing we're working with, be sure
-  // to examine `origValue` instead of `value`, if calling `toJSON` on
-  // that kind of thing would remove its identifying features -- which
-  // is to say, if that kind of thing has a `toJSON` method.
+  // When identifying what kind of thing we're working with, we
+  // examine `origValue` instead of `value`, just in case calling
+  // `toJSON` on that kind of thing would remove its identifying
+  // features -- which is to say, just in case that kind of thing has
+  // a `toJSON` method.
   //
   // For things that have a `toJSON` method, it may be convenient to
   // set `data` to `value`, if we trust that `toJSON` gives the output
   // we want to store there. And it would mean we don't discard the
   // work `JSON.stringify` did by calling `toJSON`.
   const origValue = this[key];
-  if (value instanceof ZulipVersion) {
-    return { data: value.raw(), [SERIALIZED_TYPE_FIELD_NAME]: 'ZulipVersion' };
-  } else if (origValue instanceof URL) {
-    return { data: origValue.toString(), [SERIALIZED_TYPE_FIELD_NAME]: 'URL' };
-  } else if (value instanceof GravatarURL) {
-    return { data: GravatarURL.serialize(value), [SERIALIZED_TYPE_FIELD_NAME]: 'GravatarURL' };
-  } else if (value instanceof UploadedAvatarURL) {
-    return {
-      data: UploadedAvatarURL.serialize(value),
-      [SERIALIZED_TYPE_FIELD_NAME]: 'UploadedAvatarURL',
-    };
-  } else if (value instanceof FallbackAvatarURL) {
-    return {
-      data: FallbackAvatarURL.serialize(value),
-      [SERIALIZED_TYPE_FIELD_NAME]: 'FallbackAvatarURL',
-    };
-  } else if (Immutable.Map.isMap(origValue)) {
-    return { data: value, [SERIALIZED_TYPE_FIELD_NAME]: 'ImmutableMap' };
-  }
 
-  if (typeof origValue === 'object' && origValue !== null) {
-    // Don't forget to handle a value's `toJSON` method, if present, as
-    // described above.
-    invariant(typeof origValue.toJSON !== 'function', 'unexpected toJSON');
+  if (typeof origValue !== 'object' || origValue === null) {
+    // `origValue` can't be one of our interesting data types, so,
+    // just return it.
+    return origValue;
+  }
 
-    // If storing an interesting data type, don't forget to handle it
-    // here, and in `reviver`.
-    const origValuePrototype = Object.getPrototypeOf(origValue);
-    invariant(
-      // Flow bug: https://github.com/facebook/flow/issues/6110
-      origValuePrototype === (Object.prototype: $FlowFixMe)
-        || origValuePrototype === (Array.prototype: $FlowFixMe),
-      'unexpected class',
-    );
+  switch (Object.getPrototypeOf(origValue)) {
+    // Flow bug: https://github.com/facebook/flow/issues/6110
+    case (ZulipVersion.prototype: $FlowFixMe):
+      return { data: value.raw(), [SERIALIZED_TYPE_FIELD_NAME]: 'ZulipVersion' };
+    case (URL.prototype: $FlowFixMe):
+      return { data: origValue.toString(), [SERIALIZED_TYPE_FIELD_NAME]: 'URL' };
+    case (GravatarURL.prototype: $FlowFixMe):
+      return { data: GravatarURL.serialize(value), [SERIALIZED_TYPE_FIELD_NAME]: 'GravatarURL' };
+    case (UploadedAvatarURL.prototype: $FlowFixMe):
+      return {
+        data: UploadedAvatarURL.serialize(value),
+        [SERIALIZED_TYPE_FIELD_NAME]: 'UploadedAvatarURL',
+      };
+    case (FallbackAvatarURL.prototype: $FlowFixMe):
+      return {
+        data: FallbackAvatarURL.serialize(value),
+        [SERIALIZED_TYPE_FIELD_NAME]: 'FallbackAvatarURL',
+      };
+    case (Immutable.Map.prototype: $FlowFixMe):
+      return { data: value, [SERIALIZED_TYPE_FIELD_NAME]: 'ImmutableMap' };
+    default: {
+      // If the identity of the first item in the prototype chain
+      // isn't good enough as a distinguishing mark, we can put some
+      // plain conditions here.
+    }
   }
 
-  if (typeof value === 'object' && value !== null && SERIALIZED_TYPE_FIELD_NAME in value) {
-    const copy = { ...value };
+  // Don't forget to handle a value's `toJSON` method, if present, as
+  // described above.
+  invariant(typeof origValue.toJSON !== 'function', 'unexpected toJSON');
+
+  // If storing an interesting data type, don't forget to handle it
+  // here, and in `reviver`.
+  const origValuePrototype = Object.getPrototypeOf(origValue);
+  invariant(
+    // Flow bug: https://github.com/facebook/flow/issues/6110
+    origValuePrototype === (Object.prototype: $FlowFixMe)
+      || origValuePrototype === (Array.prototype: $FlowFixMe),
+    'unexpected class',
+  );
+
+  // Ensure that objects with a [SERIALIZED_TYPE_FIELD_NAME] property
+  // round-trip.
+  if (SERIALIZED_TYPE_FIELD_NAME in origValue) {
+    const copy = { ...origValue };
     delete copy[SERIALIZED_TYPE_FIELD_NAME];
     return {
       [SERIALIZED_TYPE_FIELD_NAME]: 'Object',
       data: copy,
-      [SERIALIZED_TYPE_FIELD_NAME_ESCAPED]: value[SERIALIZED_TYPE_FIELD_NAME],
+      [SERIALIZED_TYPE_FIELD_NAME_ESCAPED]: origValue[SERIALIZED_TYPE_FIELD_NAME],
     };
   }
 
-  return value;
+  return origValue;
 };
 
+/**
+ * Custom reviver for inventive data types JSON doesn't handle.
+ *
+ * To be passed to `JSON.parse` as its second argument. New
+ * reviving logic must also appear in `replacer` so they stay in
+ * sync.
+ */
 const reviver = function reviver(key, value) {
   if (value !== null && typeof value === 'object' && SERIALIZED_TYPE_FIELD_NAME in value) {
     const data = value.data;