diff --git a/src/tw-fix-for-vanilla.js b/src/tw-fix-for-vanilla.js
index 21f635c7..34844a45 100644
--- a/src/tw-fix-for-vanilla.js
+++ b/src/tw-fix-for-vanilla.js
@@ -16,6 +16,43 @@ const removeWidthAndHeight100Percent = svgTag => {
     return false;
 };
 
+/**
+ * The paper.js used by vanilla doesn't handle rounded <rect> properly when they have only rx/ry
+ * set instead of both. While this is fixed in our paper.js fork, to ensure the project looks
+ * correct if someone imports the project into vanilla, we'll add the missing attribute.
+ * Upstream bug: https://github.com/paperjs/paper.js/issues/1863
+ * @param {SVGSVGElement} svgTag <svg> element, modified in-place.
+ * @returns {boolean} True if a change was made.
+ */
+const workaroundPaperRoundedRectangleBug = svgTag => {
+    let changed = false;
+
+    /**
+     * @param {SVGElement} element SVG element, modified in-place.
+     */
+    const recurse = element => {
+        if (element.tagName === 'rect') {
+            if (element.hasAttribute('rx') && !element.hasAttribute('ry')) {
+                changed = true;
+                element.setAttribute('ry', element.getAttribute('rx'));
+            } else if (element.hasAttribute('ry') && !element.hasAttribute('rx')) {
+                changed = true;
+                element.setAttribute('rx', element.getAttribute('ry'));
+            }
+        }
+
+        const childNodes = element.childNodes;
+        if (childNodes) {
+            for (let i = 0; i < childNodes.length; i++) {
+                recurse(childNodes[i]);
+            }
+        }
+    };
+
+    recurse(svgTag);
+    return changed;
+};
+
 /**
  * Applies fixes to an SVG to improve how it will behave in vanilla Scratch.
  * Unlike the regular loadSvgString(), this should be called once when the SVG
@@ -31,6 +68,7 @@ const fixForVanilla = rawData => {
     let changed = false;
     // To avoid short-circuiting, call the function on the left side of the ||
     changed = removeWidthAndHeight100Percent(svgTag) || changed;
+    changed = workaroundPaperRoundedRectangleBug(svgTag) || changed;
 
     if (changed) {
         const svgText = new XMLSerializer().serializeToString(svgDom);
diff --git a/test/fixtures/tw-rounded-rects.fixed.svg b/test/fixtures/tw-rounded-rects.fixed.svg
new file mode 100644
index 00000000..dfc79e0c
--- /dev/null
+++ b/test/fixtures/tw-rounded-rects.fixed.svg
@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="100" height="300">
+    <!-- should not change -->
+    <rect x="0" y="0" width="100" height="50" fill="#ff0000"/>
+
+    <!-- should add ry -->
+    <rect rx="15" x="0" y="50" width="100" height="50" fill="#00ff00" ry="15"/>
+
+    <!-- should add rx -->
+    <rect ry="100.5" x="0" y="100" width="100" height="50" fill="#0000ff" rx="100.5"/>
+
+    <!-- should not change -->
+    <rect ry="50" rx="10" x="0" y="150" width="100" height="50" fill="#00ffff"/>
+    <rect rx="22" ry="0" x="0" y="200" width="100" height="50" fill="#ff00ff"/>
+    <rect ry="0" rx="41" x="0" y="250" width="100" height="50" fill="#ffff00"/>
+</svg>
\ No newline at end of file
diff --git a/test/fixtures/tw-rounded-rects.svg b/test/fixtures/tw-rounded-rects.svg
new file mode 100644
index 00000000..3d92f096
--- /dev/null
+++ b/test/fixtures/tw-rounded-rects.svg
@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="100" height="300">
+    <!-- should not change -->
+    <rect x="0" y="0" width="100" height="50" fill="#ff0000"/>
+
+    <!-- should add ry -->
+    <rect rx="15" x="0" y="50" width="100" height="50" fill="#00ff00"/>
+
+    <!-- should add rx -->
+    <rect ry="100.5" x="0" y="100" width="100" height="50" fill="#0000ff"/>
+
+    <!-- should not change -->
+    <rect ry="50" rx="10" x="0" y="150" width="100" height="50" fill="#00ffff"/>
+    <rect rx="22" ry="0" x="0" y="200" width="100" height="50" fill="#ff00ff"/>
+    <rect ry="0" rx="41" x="0" y="250" width="100" height="50" fill="#ffff00"/>
+</svg>
\ No newline at end of file