diff --git a/src/type/unit/Unit.js b/src/type/unit/Unit.js
index 87498385b0..2943ebada7 100644
--- a/src/type/unit/Unit.js
+++ b/src/type/unit/Unit.js
@@ -1,4 +1,4 @@
-import { isComplex, isUnit, typeOf } from '../../utils/is.js'
+import { isComplex, isMatrix, isUnit, typeOf } from '../../utils/is.js'
 import { factory } from '../../utils/factory.js'
 import { memoize } from '../../utils/function.js'
 import { endsWith } from '../../utils/string.js'
@@ -1320,6 +1320,14 @@ export const createUnitClass = /* #__PURE__ */ factory(name, dependencies, ({
    * @return {Array} An array of units.
    */
   Unit.prototype.splitUnit = function (parts) {
+    // When invoked as a method in the expression parser
+    // (e.g. `(1 m).splitUnit([ft, in])`), `parts` is passed as a Matrix rather
+    // than a plain Array. Normalize it to an Array so the iteration below works
+    // in both cases.
+    if (isMatrix(parts)) {
+      parts = parts.toArray()
+    }
+
     let x = this.clone()
     const ret = []
     for (let i = 0; i < parts.length; i++) {
diff --git a/test/unit-tests/type/unit/function/splitUnit.test.js b/test/unit-tests/type/unit/function/splitUnit.test.js
index aad0b20833..078cb65cff 100644
--- a/test/unit-tests/type/unit/function/splitUnit.test.js
+++ b/test/unit-tests/type/unit/function/splitUnit.test.js
@@ -11,4 +11,13 @@ describe('splitUnit', function () {
 
     assert.strictEqual(math.evaluate('splitUnit(1 m, [ft, in])').toString(), '3 ft,3.3700787401574765 in')
   })
+
+  it('should split a unit when parts are passed as a Matrix', function () {
+    // when splitUnit is called as a method in the expression parser, the parts
+    // are passed as a Matrix rather than an Array (see #3644)
+    assert.strictEqual(splitUnit(new Unit(1, 'm'), math.matrix(['ft', 'in'])).toString(), '3 ft,3.3700787401574765 in')
+
+    assert.strictEqual(math.evaluate('(1 m).splitUnit([ft, in])').toString(), '3 ft,3.3700787401574765 in')
+    assert.strictEqual(math.evaluate('(1 m).splitUnit(["ft", "in"])').toString(), '3 ft,3.3700787401574765 in')
+  })
 })