diff --git a/.changeset/eleven-taxis-hear.md b/.changeset/eleven-taxis-hear.md
new file mode 100644
index 000000000..f75fcdc90
--- /dev/null
+++ b/.changeset/eleven-taxis-hear.md
@@ -0,0 +1,5 @@
+---
+'slate': patch
+---
+
+Fixed flaw in deep-equal algorithm when dealing with nested mark objects
diff --git a/packages/slate/src/utils/deep-equal.ts b/packages/slate/src/utils/deep-equal.ts
index 623474c46..027057909 100644
--- a/packages/slate/src/utils/deep-equal.ts
+++ b/packages/slate/src/utils/deep-equal.ts
@@ -17,7 +17,7 @@ export const isDeepEqual = (
   for (const key in node) {
     const a = node[key]
     const b = another[key]
-    if (isPlainObject(a)) {
+    if (isPlainObject(a) && isPlainObject(b)) {
       if (!isDeepEqual(a, b)) return false
     } else if (Array.isArray(a) && Array.isArray(b)) {
       if (a.length !== b.length) return false
diff --git a/packages/slate/test/utils/deep-equal/deep-not-equal-nested-undefined.js b/packages/slate/test/utils/deep-equal/deep-not-equal-nested-undefined.js
new file mode 100644
index 000000000..b5a26d47d
--- /dev/null
+++ b/packages/slate/test/utils/deep-equal/deep-not-equal-nested-undefined.js
@@ -0,0 +1,21 @@
+import { isDeepEqual } from '../../../src/utils/deep-equal'
+
+export const input = {
+  objectA: {
+    text: 'same text',
+    bold: true,
+    italic: { origin: 'inherited', value: true },
+    underline: { origin: 'inherited', value: false },
+  },
+  objectB: {
+    text: 'same text',
+    bold: true,
+    italic: { origin: 'inherited', value: true },
+  },
+}
+
+export const test = ({ objectA, objectB }) => {
+  return isDeepEqual(objectA, objectB)
+}
+
+export const output = false