diff --git a/src/stringify.js b/src/stringify.js
index df291fd..0904fc7 100644
--- a/src/stringify.js
+++ b/src/stringify.js
@@ -44,11 +44,21 @@ export function stringify(value, reducers) {
 
 	/** @param {any} thing */
 	function flatten(thing) {
-		if (typeof thing === 'function') {
-			throw new DevalueError(`Cannot stringify a function`, keys);
+		if (indexes.has(thing)) return indexes.get(thing);
+
+    for (const { key, fn } of custom) {
+			const value = fn(thing);
+			if (value) {
+        const index = p++;
+        indexes.set(thing, index);
+				stringified[index] = `["${key}",${flatten(value)}]`;
+				return index;
+			}
 		}
 
-		if (indexes.has(thing)) return indexes.get(thing);
+    if (typeof thing === 'function') {
+			throw new DevalueError(`Cannot stringify a function`, keys);
+		}
 
 		if (thing === undefined) return UNDEFINED;
 		if (Number.isNaN(thing)) return NAN;
@@ -59,14 +69,6 @@ export function stringify(value, reducers) {
 		const index = p++;
 		indexes.set(thing, index);
 
-		for (const { key, fn } of custom) {
-			const value = fn(thing);
-			if (value) {
-				stringified[index] = `["${key}",${flatten(value)}]`;
-				return index;
-			}
-		}
-
 		let str = '';
 
 		if (is_primitive(thing)) {
@@ -157,16 +159,16 @@ export function stringify(value, reducers) {
 					str = '["' + type + '","' + base64 + '"]';
 					break;
 				}
-					
+
 				case "ArrayBuffer": {
 					/** @type {ArrayBuffer} */
 					const arraybuffer = thing;
 					const base64 = encode64(arraybuffer);
-					
+
 					str = `["ArrayBuffer","${base64}"]`;
 					break;
 				}
-				
+
 				default:
 					if (!is_plain_object(thing)) {
 						throw new DevalueError(
diff --git a/test/test.js b/test/test.js
index ebd4147..279ba1e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -442,12 +442,36 @@ const fixtures = {
 				assert.equal(obj1.value.answer, 42);
 			}
 		}
-	])(new Custom({ answer: 42 }))
+	])(new Custom({ answer: 42 })),
+
+	customFunction: ((func) => [
+		{
+			name: 'Custom function type',
+			value: func,
+			json: '[["function",1],"func"]',
+			reducers: {
+				function: (x) => x === func && 'func'
+			},
+			revivers: {
+        function: (x) => {
+          assert.is(x, 'func');
+          return func;
+        }
+			},
+			validate: (f) => {
+				assert.is(f, func);
+			}
+		}
+	])(() => 42)
 };
 
 for (const [name, tests] of Object.entries(fixtures)) {
 	const test = uvu.suite(`uneval: ${name}`);
 	for (const t of tests) {
+    if (!t.js) {
+      continue;
+    }
+
 		test(t.name, () => {
 			const actual = uneval(t.value, t.replacer);
 			const expected = t.js;