diff --git a/lib/utils.js b/lib/utils.js
index 75f3d111..c243ebb4 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -26,7 +26,7 @@ export function PDFNumber(n) {
  * - Or `{top: SideValue, right: SideValue, bottom: SideValue, left: SideValue}`
  *
  * @template T
- * @typedef {T | [T, T] | [T, T, T, T] | { vertical: T; horizontal: T } | { top: T; right: T; bottom: T; left: T }} SideDefinition<T>
+ * @typedef {T | [T, T] | [T, T, T, T] | { vertical: T; horizontal: T } | ExpandedSideDefinition<T>} SideDefinition<T>
  **/
 
 /**
@@ -52,12 +52,13 @@ export function normalizeSides(
   transformer = (v) => v,
 ) {
   if (
-    sides === undefined || sides === null ||
+    sides === undefined ||
+    sides === null ||
     (typeof sides === "object" && Object.keys(sides).length === 0)
   ) {
     sides = defaultDefinition;
   }
-  if (typeof sides === "string" || typeof sides === "number" || !sides) {
+  if (typeof sides !== "object" || sides === null) {
     sides = [sides, sides, sides, sides];
   }
   if (Array.isArray(sides)) {
diff --git a/tests/unit/utils.spec.js b/tests/unit/utils.spec.js
index b39be48f..9f0fc717 100644
--- a/tests/unit/utils.spec.js
+++ b/tests/unit/utils.spec.js
@@ -33,6 +33,14 @@ describe("normalizeSides", () => {
       undefined,
       { top: undefined, right: undefined, bottom: undefined, left: undefined },
     ],
+    [
+      true,
+      { top: true, right: true, bottom: true, left: true },
+    ],
+    [
+      false,
+      { top: false, right: false, bottom: false, left: false },
+    ],
   ])("%s -> %s", (size, expected) => {
     expect(normalizeSides(size)).toEqual(expected);
   });