diff --git a/bg/monitor.js b/bg/monitor.js
index 7ac04af..e4616a3 100644
--- a/bg/monitor.js
+++ b/bg/monitor.js
@@ -22,7 +22,7 @@ chrome.storage.onChanged.addListener((changes, area) => {
 
 function handleUpdated(tabId, changeInfo, tabInfo) {
     //toggle tab if it is detected by the filter
-    if (tabInfo.url.includes(options.urlContains) === false) {
+    if (tabInfo.url.includes(encodeURI(options.urlContains)) === false) {
         return;
     }
     // console.log("Matched tab url", tabInfo.url);
diff --git a/options.html b/options.html
index 430bfee..3cc01cf 100644
--- a/options.html
+++ b/options.html
@@ -26,7 +26,8 @@
             </div>
             <div class="pure-control-group">
                 <label for="url-contains">Detect url containing </label>
-                <input id="url-contains" type="text" title="Any url that contains this string will be automatically detected." />
+                <input id="url-contains" type="text" title="Any url that contains this string will be automatically detected.
+(Some characters are automatically encoded to match the url spec. Like 'Space' -> '%20'.)" />
             </div>
             <div class="pure-control-group">
                 <label for="prepend-instead">Use prepend instead of append </label>