diff --git a/lighthouse-core/audits/byte-efficiency/byte-efficiency-audit.js b/lighthouse-core/audits/byte-efficiency/byte-efficiency-audit.js
index 26cc33c161a6..333fdd05e4db 100644
--- a/lighthouse-core/audits/byte-efficiency/byte-efficiency-audit.js
+++ b/lighthouse-core/audits/byte-efficiency/byte-efficiency-audit.js
@@ -93,8 +93,10 @@ class UnusedBytes extends Audit {
       // This was an asset that was inlined in a different resource type (e.g. HTML document).
       // Use the compression ratio of the resource to estimate the total transferred bytes.
       const transferSize = networkRecord.transferSize || 0;
-      const resourceSize = networkRecord.resourceSize;
-      const compressionRatio = resourceSize !== undefined ? (transferSize / resourceSize) : 1;
+      const resourceSize = networkRecord.resourceSize || 0;
+      // Get the compression ratio, if it's an invalid number, assume no compression.
+      const compressionRatio = Number.isFinite(resourceSize) && resourceSize > 0 ?
+        (transferSize / resourceSize) : 1;
       return Math.round(totalBytes * compressionRatio);
     }
   }
diff --git a/lighthouse-core/audits/byte-efficiency/unused-css-rules.js b/lighthouse-core/audits/byte-efficiency/unused-css-rules.js
index d2b7344d7a8f..2bb3d3a6b1b9 100644
--- a/lighthouse-core/audits/byte-efficiency/unused-css-rules.js
+++ b/lighthouse-core/audits/byte-efficiency/unused-css-rules.js
@@ -47,6 +47,10 @@ class UnusedCSSRules extends ByteEfficiencyAudit {
    */
   static indexStylesheetsById(styles, networkRecords) {
     const indexedNetworkRecords = networkRecords
+        // Some phantom network records appear with a 0 resourceSize that aren't real.
+        // A network record that has no size data is just as good as no network record at all for our
+        // purposes, so we'll just filter them out. https://github.com/GoogleChrome/lighthouse/issues/9684#issuecomment-532381611
+        .filter(record => record.resourceSize > 0)
         .reduce((indexed, record) => {
           indexed[record.url] = record;
           return indexed;
diff --git a/lighthouse-core/test/audits/byte-efficiency/byte-efficiency-audit-test.js b/lighthouse-core/test/audits/byte-efficiency/byte-efficiency-audit-test.js
index b6656de7861e..103ee81879dc 100644
--- a/lighthouse-core/test/audits/byte-efficiency/byte-efficiency-audit-test.js
+++ b/lighthouse-core/test/audits/byte-efficiency/byte-efficiency-audit-test.js
@@ -77,6 +77,12 @@ describe('Byte efficiency base audit', () => {
       const result = estimate({transferSize: 1000, resourceType}, 100);
       assert.equal(result, 100);
     });
+
+    it('should not error when resource size is 0', () => {
+      const resourceType = 'Other';
+      const result = estimate({transferSize: 1000, resourceSize: 0, resourceType}, 100);
+      assert.equal(result, 100);
+    });
   });
 
   it('should format details', () => {
diff --git a/lighthouse-core/test/audits/byte-efficiency/unused-css-rules-test.js b/lighthouse-core/test/audits/byte-efficiency/unused-css-rules-test.js
index ff3c7c23690c..be8b64b281e1 100644
--- a/lighthouse-core/test/audits/byte-efficiency/unused-css-rules-test.js
+++ b/lighthouse-core/test/audits/byte-efficiency/unused-css-rules-test.js
@@ -122,6 +122,7 @@ describe('Best Practices: unused css rules audit', () => {
       {
         url: 'file://a.css',
         transferSize: 100 * 1024,
+        resourceSize: 100 * 1024,
         resourceType: 'Stylesheet',
       },
     ];
@@ -191,6 +192,36 @@ describe('Best Practices: unused css rules audit', () => {
       });
     });
 
+    it('handles phantom network records without size data', async () => {
+      const result = await UnusedCSSAudit.audit_(getArtifacts({
+        CSSUsage: {rules: [
+          {styleSheetId: 'a', used: true, startOffset: 0, endOffset: 60000}, // 40000 * 3 * 50% = 60000
+        ], stylesheets: [
+          {
+            header: {styleSheetId: 'a', sourceURL: 'file://a.html'},
+            content: `${generate('123', 40000)}`, // stylesheet size of 40000 * 3 uncompressed bytes
+          },
+        ]},
+      }), [
+        {
+          url: 'file://a.html',
+          transferSize: 100 * 1024 * 0.5, // compression ratio of 0.5
+          resourceSize: 100 * 1024,
+          resourceType: 'Document', // this is a document so it'll use the compressionRatio but not the raw size
+        },
+        {
+          url: 'file://a.html',
+          transferSize: 0,
+          resourceSize: 0,
+          resourceType: 'Document',
+        },
+      ]);
+
+      expect(result.items).toMatchObject([
+        {totalBytes: 40000 * 3 * 0.5, wastedPercent: 50},
+      ]);
+    });
+
     it('does not include empty or small sheets', () => {
       return UnusedCSSAudit.audit_(getArtifacts({
         CSSUsage: {rules: [