diff --git a/browser/components/sessionstore/src/nsSessionStartup.js b/browser/components/sessionstore/src/nsSessionStartup.js
index afe924288cc8..95403c25ab1c 100644
--- a/browser/components/sessionstore/src/nsSessionStartup.js
+++ b/browser/components/sessionstore/src/nsSessionStartup.js
@@ -126,15 +126,16 @@ SessionStartup.prototype = {
       return;
     }
 
-    this._initialState =  this._parseStateString(stateString);
+    this._initialState = this._parseStateString(stateString);
 
     let shouldResumeSessionOnce = Services.prefs.getBoolPref("browser.sessionstore.resume_session_once");
     let shouldResumeSession = shouldResumeSessionOnce ||
           Services.prefs.getIntPref("browser.startup.page") == BROWSER_STARTUP_RESUME_SESSION;
 
     // If this is a normal restore then throw away any previous session
-    if (!shouldResumeSessionOnce)
+    if (!shouldResumeSessionOnce && this._initialState) {
       delete this._initialState.lastSessionState;
+    }
 
     let resumeFromCrash = Services.prefs.getBoolPref("browser.sessionstore.resume_from_crash");
 
diff --git a/browser/components/sessionstore/test/unit/data/sessionstore_invalid.js b/browser/components/sessionstore/test/unit/data/sessionstore_invalid.js
new file mode 100644
index 000000000000..a8c3ff2ff9cf
--- /dev/null
+++ b/browser/components/sessionstore/test/unit/data/sessionstore_invalid.js
@@ -0,0 +1,3 @@
+{
+  "windows": // invalid json
+}
diff --git a/browser/components/sessionstore/test/unit/test_startup_invalid_session.js b/browser/components/sessionstore/test/unit/test_startup_invalid_session.js
new file mode 100644
index 000000000000..9f6df8585888
--- /dev/null
+++ b/browser/components/sessionstore/test/unit/test_startup_invalid_session.js
@@ -0,0 +1,21 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function run_test() {
+  let profd = do_get_profile();
+
+  let sourceSession = do_get_file("data/sessionstore_invalid.js");
+  sourceSession.copyTo(profd, "sessionstore.js");
+
+  let sourceCheckpoints = do_get_file("data/sessionCheckpoints_all.json");
+  sourceCheckpoints.copyTo(profd, "sessionCheckpoints.json");
+
+  do_test_pending();
+  let startup = Cc["@mozilla.org/browser/sessionstartup;1"].
+    getService(Ci.nsISessionStartup);
+
+  afterSessionStartupInitialization(function cb() {
+    do_check_eq(startup.sessionType, Ci.nsISessionStartup.NO_SESSION);
+    do_test_finished();
+  });
+}
diff --git a/browser/components/sessionstore/test/unit/xpcshell.ini b/browser/components/sessionstore/test/unit/xpcshell.ini
index 550052056a6a..b2dcb0b8c2df 100644
--- a/browser/components/sessionstore/test/unit/xpcshell.ini
+++ b/browser/components/sessionstore/test/unit/xpcshell.ini
@@ -4,9 +4,11 @@ tail =
 firefox-appdir = browser
 support-files =
   data/sessionCheckpoints_all.json
+  data/sessionstore_invalid.js
   data/sessionstore_valid.js
 
 [test_backup.js]
 [test_backup_once.js]
 [test_startup_nosession_async.js]
 [test_startup_session_async.js]
+[test_startup_invalid_session.js]