--- a/browser/components/sessionstore/test/browser_524745.js
+++ b/browser/components/sessionstore/test/browser_524745.js
@@ -18,17 +18,17 @@ function test() {
       let selectedWindow = state.windows[state.selectedWindow - 1];
       is(selectedWindow.extData && selectedWindow.extData[uniqKey], uniqVal,
          "selectedWindow is window_B");
 
       // Now minimize window_B. The selected window shouldn't have the secret data
       window_B.minimize();
       waitForFocus(function() {
         state = JSON.parse(ss.getBrowserState());
-        selectedWindow = state.windows[state.selectedWindow - 1];
+        selectedWindow = state.windows[Math.ceil(state.selectedWindow - 1, 0)];
         ok(!selectedWindow.extData || !selectedWindow.extData[uniqKey],
            "selectedWindow is not window_B after minimizing it");
 
         // Now minimize the last open window (assumes no other tests left windows open)
         window.minimize();
         state = JSON.parse(ss.getBrowserState());
         is(state.selectedWindow, 0,
            "selectedWindow should be 0 when all windows are minimized");