diff --git a/lib/index.d.ts b/lib/index.d.ts
index 764c435..be8b1f3 100644
--- a/lib/index.d.ts
+++ b/lib/index.d.ts
@@ -25,8 +25,8 @@ declare namespace postcssValueParser {
     before: string;
 
     /**
-    * The token at the end of the node
-    */
+     * The token at the end of the node
+     */
     after: string;
   }
 
@@ -85,7 +85,7 @@ declare namespace postcssValueParser {
      * @param node The node to stringify
      * @returns The serialized CSS representation of the node
      */
-    (nodes: Node): string;
+    (nodes: Node): string | undefined;
   }
 
   interface WalkCallback {
@@ -95,7 +95,7 @@ declare namespace postcssValueParser {
      * @param nodes The series of parsed nodes
      * @returns Returning `false` will prevent traversal of descendant nodes (only applies if `bubble` was set to `true` in the `walk()` call)
      */
-    (node: Node, index: number, nodes: Node[]): void | false;
+    (node: Node, index: number, nodes: Node[]): void | boolean;
   }
 
   /**
diff --git a/lib/stringify.js b/lib/stringify.js
index 5f2c845..6079671 100644
--- a/lib/stringify.js
+++ b/lib/stringify.js
@@ -16,7 +16,7 @@ function stringifyNode(node, custom) {
   } else if (type === "div") {
     return (node.before || "") + value + (node.after || "");
   } else if (Array.isArray(node.nodes)) {
-    buf = stringify(node.nodes);
+    buf = stringify(node.nodes, custom);
     if (type !== "function") {
       return buf;
     }
diff --git a/package.json b/package.json
index 44e04cd..55789ae 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "postcss-value-parser",
-  "version": "4.0.3",
+  "version": "4.1.0",
   "description": "Transforms css values and at-rule params into the tree",
   "main": "lib/index.js",
   "files": [
@@ -15,7 +15,7 @@
     "tape": "^4.10.2"
   },
   "scripts": {
-    "lint:prettier": "prettier '**/*.js' --list-different",
+    "lint:prettier": "prettier '**/*.js' '**/*.ts' --list-different",
     "lint:js": "eslint . --cache",
     "lint": "yarn lint:js && yarn lint:prettier",
     "pretest": "yarn lint",
diff --git a/test/stringify.js b/test/stringify.js
index fb695a5..36d13f2 100644
--- a/test/stringify.js
+++ b/test/stringify.js
@@ -89,7 +89,7 @@ var tests = [
 ];
 
 test("Stringify", function(t) {
-  t.plan(tests.length + 3);
+  t.plan(tests.length + 4);
 
   tests.forEach(function(opts) {
     t.equal(stringify(parse(opts.fixture)), opts.fixture, opts.message);
@@ -131,4 +131,13 @@ test("Stringify", function(t) {
 
   tokens[1].type = "word";
   t.equal(stringify(tokens), " rgba ", "Shouldn't process nodes of work type");
+
+  t.equal(
+    stringify(parse("calc(1px + var(--bar))"), function(node) {
+      if (node.type === "function" && node.value === "var") {
+        return "10px";
+      }
+    }),
+    "calc(1px + 10px)"
+  );
 });