diff --git a/toolkit/components/prompts/src/nsPrompter.js b/toolkit/components/prompts/src/nsPrompter.js
index 02ad0b21d09..19cfa7254b3 100644
--- a/toolkit/components/prompts/src/nsPrompter.js
+++ b/toolkit/components/prompts/src/nsPrompter.js
@@ -314,7 +314,7 @@ let PromptUtils = {
 
         // Suppress "the site says: $realm" when we synthesized a missing realm.
         if (!authInfo.realm && !isProxy)
-            realm = null;
+            realm = "";
 
         // Trim obnoxiously long realms.
         if (realm.length > 150) {
diff --git a/toolkit/components/prompts/test/test_modal_prompts.html b/toolkit/components/prompts/test/test_modal_prompts.html
index 54e87068eb3..4299bd4fccf 100644
--- a/toolkit/components/prompts/test/test_modal_prompts.html
+++ b/toolkit/components/prompts/test/test_modal_prompts.html
@@ -22,6 +22,8 @@ netscape.security.PrivilegeManager.enablePrivilege('UniversalXPConnect');
 
 let prompter = Cc["@mozilla.org/embedcomp/prompt-service;1"].
                getService(Ci.nsIPromptService2);
+let ioService = Cc["@mozilla.org/network/io-service;1"].
+                getService(Ci.nsIIOService);
 
 function checkExpectedState(doc, state) {
     let msg        = doc.getElementById("info.body").textContent;
@@ -635,6 +637,55 @@ function handleDialog(doc, testNum) {
         clickOK = false;
         break;
 
+      case 100:
+        // PromptAuth (no realm, ok, with checkbox)
+        state = {
+            msg : 'Enter username and password for http://example.com',
+            title : "TestTitle",
+            iconClass   : "authentication-icon question-icon",
+            textHidden  : false,
+            passHidden  : false,
+            checkHidden : false,
+            textValue   : "",
+            passValue   : "",
+            checkMsg    : "Check me out!",
+            checked     : false,
+        };
+        checkExpectedState(doc, state);
+
+        textField.setAttribute("value", "username");
+        passField.setAttribute("value", "password");
+        // XXX dumb. old code driven by oncommand.
+        checkbox.setChecked(true);
+        checkbox.doCommand();
+        break;
+
+      case 101:
+        // PromptAuth (long realm, ok, with checkbox)
+        state = {
+            msg : 'A username and password are being requested by http://example.com. The site '  +
+                  'says: "abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi ' +
+                  'abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi abcdefghi '        +
+                  'abcdefghi \u2026"',
+            title : "TestTitle",
+            iconClass   : "authentication-icon question-icon",
+            textHidden  : false,
+            passHidden  : false,
+            checkHidden : false,
+            textValue   : "",
+            passValue   : "",
+            checkMsg    : "Check me out!",
+            checked     : false,
+        };
+        checkExpectedState(doc, state);
+
+        textField.setAttribute("value", "username");
+        passField.setAttribute("value", "password");
+        // XXX dumb. old code driven by oncommand.
+        checkbox.setChecked(true);
+        checkbox.doCommand();
+        break;
+
       default:
         ok(false, "Uhh, unhandled switch for testNum #" + testNum);
         break;
@@ -973,7 +1024,50 @@ is(checkVal.value, true, "expected checkbox setting");
 ok(didDialog, "handleDialog was invoked");
 
 
-// promptAuth already tested via password manager
+// promptAuth already tested via password manager but do a few specific things here.
+
+
+var channel = ioService.newChannel("http://example.com", null, null);
+var level = Ci.nsIAuthPrompt2.LEVEL_NONE;
+var authinfo = {
+    username : "",
+    password : "",
+    domain   : "",
+
+    flags : Ci.nsIAuthInformation.AUTH_HOST,
+    authenticationScheme : "basic",
+    realm : ""
+};
+
+// ===== test 100 =====
+// promptAuth with empty realm
+testNum = 100;
+startCallbackTimer();
+checkVal.value = false;
+isOK = prompter.promptAuth(window, channel, level, authinfo, "Check me out!", checkVal);
+is(isOK, true, "checked expected retval");
+is(authinfo.username, "username", "checking filled username");
+is(authinfo.password, "password", "checking filled password");
+is(checkVal.value, true, "expected checkbox setting");
+ok(didDialog, "handleDialog was invoked");
+
+// ===== test 101 =====
+// promptAuth with long realm
+testNum++;
+startCallbackTimer();
+checkVal.value = false;
+var longString = "";
+for (var i = 0; i < 20; i++)
+    longString += "abcdefghi "; // 200 chars long
+authinfo.realm = longString;
+authinfo.username = "";
+authinfo.password = "";
+isOK = prompter.promptAuth(window, channel, level, authinfo, "Check me out!", checkVal);
+is(isOK, true, "checked expected retval");
+is(authinfo.username, "username", "checking filled username");
+is(authinfo.password, "password", "checking filled password");
+is(checkVal.value, true, "expected checkbox setting");
+ok(didDialog, "handleDialog was invoked");
 
 </script>
 </pre>