diff --git a/src/utils.js b/src/utils.js
index 5dab80a..3b242b8 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -119,7 +119,11 @@ function getUrlObject(req) {
  */
 export function getPublicUrl(publicUrl, req) {
   if (publicUrl) {
-    return publicUrl;
+    try {
+      return new URL(publicUrl).toString();
+    } catch {
+      return new URL(publicUrl, getUrlObject(req)).toString();
+    }
   }
   return getUrlObject(req).toString();
 }
diff --git a/test/style.js b/test/style.js
index 469eab7..f2af4fb 100644
--- a/test/style.js
+++ b/test/style.js
@@ -25,7 +25,7 @@ describe('Styles', function () {
           expect(res.body.sources).to.be.a('object');
           expect(res.body.glyphs).to.be.a('string');
           expect(res.body.sprite).to.be.a('string');
-          expect(res.body.sprite).to.be.equal('/test/styles/test-style/sprite');
+          expect(res.body.sprite).to.contain('/test/styles/test-style/sprite');
           expect(res.body.layers).to.be.a('array');
         })
         .end(done);