diff --git a/src/serialization/sb3.js b/src/serialization/sb3.js
index df367cf1b..ef5010a90 100644
--- a/src/serialization/sb3.js
+++ b/src/serialization/sb3.js
@@ -14,6 +14,7 @@ const StageLayering = require('../engine/stage-layering');
 const log = require('../util/log');
 const uid = require('../util/uid');
 const MathUtil = require('../util/math-util');
+const StringUtil = require('../util/string-util');
 
 const {loadCostume} = require('../import/load-costume.js');
 const {loadSound} = require('../import/load-sound.js');
@@ -522,7 +523,7 @@ const serialize = function (runtime, targetId) {
 
     const layerOrdering = getSimplifiedLayerOrdering(originalTargetsToSerialize);
 
-    const flattenedOriginalTargets = JSON.parse(JSON.stringify(originalTargetsToSerialize));
+    const flattenedOriginalTargets = JSON.parse(StringUtil.stringify(originalTargetsToSerialize));
 
     // If the renderer is attached, and we're serializing a whole project (not a sprite)
     // add a temporary layerOrder property to each target.
diff --git a/src/util/string-util.js b/src/util/string-util.js
index 1d8f18d4f..c308cc6c8 100644
--- a/src/util/string-util.js
+++ b/src/util/string-util.js
@@ -34,7 +34,28 @@ class StringUtil {
             return [text.substring(0, index), text.substring(index + 1)];
         }
         return [text, null];
-        
+
+    }
+
+    /**
+     * A customized version of JSON.stringify that sets Infinity/NaN to 0,
+     * instead of the default (null).
+     * Needed because null is not of type number, but Infinity/NaN are, which
+     * can lead to serialization producing JSON that isn't valid based on the parser schema.
+     * It is also consistent with the behavior of saving 2.0 projects.
+     * This is only needed when stringifying an object for saving.
+     *
+     * @param {!object} obj - The object to serialize
+     * @return {!string} The JSON.stringified string with Infinity/NaN replaced with 0
+     */
+    static stringify (obj) {
+        return JSON.stringify(obj, (_key, value) => {
+            if (typeof value === 'number' &&
+               (value === Infinity || value === -Infinity || isNaN(value))){
+                return 0;
+            }
+            return value;
+        });
     }
 }
 
diff --git a/src/virtual-machine.js b/src/virtual-machine.js
index 2bc8eb0d4..e25f757a1 100644
--- a/src/virtual-machine.js
+++ b/src/virtual-machine.js
@@ -381,7 +381,7 @@ class VirtualMachine extends EventEmitter {
     exportSprite (targetId, optZipType) {
         const soundDescs = serializeSounds(this.runtime, targetId);
         const costumeDescs = serializeCostumes(this.runtime, targetId);
-        const spriteJson = JSON.stringify(sb3.serialize(this.runtime, targetId));
+        const spriteJson = StringUtil.stringify(sb3.serialize(this.runtime, targetId));
 
         const zip = new JSZip();
         zip.file('sprite.json', spriteJson);
@@ -401,7 +401,7 @@ class VirtualMachine extends EventEmitter {
      * @return {string} Serialized state of the runtime.
      */
     toJSON () {
-        return JSON.stringify(sb3.serialize(this.runtime));
+        return StringUtil.stringify(sb3.serialize(this.runtime));
     }
 
     // TODO do we still need this function? Keeping it here so as not to introduce
diff --git a/test/unit/util_string.js b/test/unit/util_string.js
index aca20f9a4..dd2fb2339 100644
--- a/test/unit/util_string.js
+++ b/test/unit/util_string.js
@@ -63,3 +63,14 @@ test('unusedName', t => {
     );
     t.end();
 });
+
+test('stringify', t => {
+    const obj = {a: Infinity, b: NaN, c: -Infinity, d: 23, e: 'str'};
+    const parsed = JSON.parse(StringUtil.stringify(obj));
+    t.equal(parsed.a, 0);
+    t.equal(parsed.b, 0);
+    t.equal(parsed.c, 0);
+    t.equal(parsed.d, 23);
+    t.equal(parsed.e, 'str');
+    t.end();
+});
diff --git a/test/unit/virtual-machine.js b/test/unit/virtual-machine.js
index 78036fdcf..6bac07f17 100644
--- a/test/unit/virtual-machine.js
+++ b/test/unit/virtual-machine.js
@@ -982,3 +982,30 @@ test('Starting the VM emits an event', t => {
     t.equal(started, true);
     t.end();
 });
+
+test('toJSON encodes Infinity/NaN as 0, not null', t => {
+    const vm = new VirtualMachine();
+    const runtime = vm.runtime;
+    const spr1 = new Sprite(null, runtime);
+    const stage = spr1.createClone();
+    stage.isStage = true;
+    stage.volume = Infinity;
+    stage.tempo = NaN;
+    stage.createVariable('id1', 'name1', '');
+    stage.variables.id1.value = Infinity;
+    stage.createVariable('id2', 'name2', '');
+    stage.variables.id1.value = -Infinity;
+    stage.createVariable('id3', 'name3', '');
+    stage.variables.id1.value = NaN;
+
+    runtime.targets = [stage];
+
+    const json = JSON.parse(vm.toJSON());
+    t.equal(json.targets[0].volume, 0);
+    t.equal(json.targets[0].tempo, 0);
+    t.equal(json.targets[0].variables.id1[1], 0);
+    t.equal(json.targets[0].variables.id2[1], 0);
+    t.equal(json.targets[0].variables.id3[1], 0);
+
+    t.end();
+});