diff --git a/platform/chromium/vapi-client.js b/platform/chromium/vapi-client.js
index c0f6336f1..8e659499d 100644
--- a/platform/chromium/vapi-client.js
+++ b/platform/chromium/vapi-client.js
@@ -57,7 +57,7 @@ if ( /^image\/|^text\/plain/.test(contentType) ) {
 /******************************************************************************/
 
 // https://bugs.chromium.org/p/project-zero/issues/detail?id=1225&desc=6#c10
-if ( !self.vAPI || !self.vAPI.uBO ) {
+if ( !self.vAPI || self.vAPI.uBO !== true ) {
     self.vAPI = { uBO: true };
 }
 
diff --git a/platform/chromium/vapi-common.js b/platform/chromium/vapi-common.js
index a92fe3f4c..5a2c27993 100644
--- a/platform/chromium/vapi-common.js
+++ b/platform/chromium/vapi-common.js
@@ -29,7 +29,7 @@
 (function(self) {
 
 // https://bugs.chromium.org/p/project-zero/issues/detail?id=1225&desc=6#c10
-if ( !self.vAPI || !self.vAPI.uBO ) {
+if ( !self.vAPI || self.vAPI.uBO !== true ) {
     self.vAPI = { uBO: true };
 }
 
diff --git a/platform/firefox/vapi-client.js b/platform/firefox/vapi-client.js
index e810d79d8..606f36f12 100644
--- a/platform/firefox/vapi-client.js
+++ b/platform/firefox/vapi-client.js
@@ -46,7 +46,7 @@ if ( document instanceof HTMLDocument === false ) {
 /******************************************************************************/
 
 // https://bugs.chromium.org/p/project-zero/issues/detail?id=1225&desc=6#c10
-if ( !self.vAPI || !self.vAPI.uBO ) {
+if ( !self.vAPI || self.vAPI.uBO !== true ) {
     self.vAPI = { uBO: true };
 }
 
diff --git a/platform/firefox/vapi-common.js b/platform/firefox/vapi-common.js
index 6b0200d18..1ee769987 100644
--- a/platform/firefox/vapi-common.js
+++ b/platform/firefox/vapi-common.js
@@ -37,7 +37,7 @@ const {Services} = Components.utils.import(
 );
 
 // https://bugs.chromium.org/p/project-zero/issues/detail?id=1225&desc=6#c10
-if ( !self.vAPI || !self.vAPI.uBO ) {
+if ( !self.vAPI || self.vAPI.uBO !== true ) {
     self.vAPI = { uBO: true };
 }