diff --git a/js/desktop/src/lib/avatar-image.ts b/js/desktop/src/lib/avatar-image.ts
index 2fc7aac..6bc9b79 100644
--- a/js/desktop/src/lib/avatar-image.ts
+++ b/js/desktop/src/lib/avatar-image.ts
@@ -10,6 +10,9 @@ export async function toAvatarBlob(
   source: ImageBitmap,
   { size = 512, mirror = false }: { size?: number; mirror?: boolean } = {},
 ): Promise<Blob> {
+  if (!Number.isInteger(size) || size <= 0) {
+    throw new Error('Avatar size must be a positive integer');
+  }
   const side = Math.min(source.width, source.height);
   if (side === 0) {
     // A not-yet-decoded <video> or a corrupt image yields a zero-size source;