diff --git a/mozilla/accessible/src/html/nsHTMLTableAccessible.cpp b/mozilla/accessible/src/html/nsHTMLTableAccessible.cpp
index 8d677f51233..fb5b9011349 100644
--- a/mozilla/accessible/src/html/nsHTMLTableAccessible.cpp
+++ b/mozilla/accessible/src/html/nsHTMLTableAccessible.cpp
@@ -58,6 +58,7 @@
 #include "nsIServiceManager.h"
 #include "nsITableLayout.h"
 #include "nsITableCellLayout.h"
+#include "nsLayoutErrors.h"
 
 
 NS_IMPL_ISUPPORTS_INHERITED0(nsHTMLTableCellAccessible, nsHyperTextAccessible)
@@ -662,11 +663,15 @@ nsHTMLTableAccessible::IsRowSelected(PRInt32 aRow, PRBool *_retval)
 
 NS_IMETHODIMP
 nsHTMLTableAccessible::IsCellSelected(PRInt32 aRow, PRInt32 aColumn,
-                                      PRBool *_retval)
+                                      PRBool *aIsSelected)
 {
-  NS_ENSURE_TRUE(IsValidRow(aRow) && IsValidColumn(aColumn), NS_ERROR_INVALID_ARG);
+  NS_ENSURE_ARG_POINTER(aIsSelected);
+  *aIsSelected = PR_FALSE;
 
-  nsITableLayout *tableLayout;
+  NS_ENSURE_TRUE(IsValidRow(aRow) && IsValidColumn(aColumn),
+                 NS_ERROR_INVALID_ARG);
+
+  nsITableLayout *tableLayout = nsnull;
   nsresult rv = GetTableLayout(&tableLayout);
   NS_ENSURE_SUCCESS(rv, rv);
 
@@ -674,11 +679,14 @@ nsHTMLTableAccessible::IsCellSelected(PRInt32 aRow, PRInt32 aColumn,
   PRInt32 startRowIndex = 0, startColIndex = 0,
           rowSpan, colSpan, actualRowSpan, actualColSpan;
 
-  return tableLayout->GetCellDataAt(aRow, aColumn,
-                                    *getter_AddRefs(domElement),
-                                    startRowIndex, startColIndex, rowSpan,
-                                    colSpan, actualRowSpan, actualColSpan,
-                                    *_retval);
+  rv = tableLayout->GetCellDataAt(aRow, aColumn, *getter_AddRefs(domElement),
+                                  startRowIndex, startColIndex,
+                                  rowSpan, colSpan,
+                                  actualRowSpan, actualColSpan, *aIsSelected);
+
+  if (rv == NS_TABLELAYOUT_CELL_NOT_FOUND)
+    return NS_ERROR_INVALID_ARG;
+  return rv;
 }
 
 PRBool
@@ -867,15 +875,18 @@ nsHTMLTableAccessible::GetCellAt(PRInt32        aRowIndex,
           rowSpan, colSpan, actualRowSpan, actualColSpan;
   PRBool isSelected;
 
-  nsITableLayout *tableLayout;
+  nsITableLayout *tableLayout = nsnull;
   nsresult rv = GetTableLayout(&tableLayout);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  return tableLayout->GetCellDataAt(aRowIndex, aColIndex, aCell,
-                                    startRowIndex, startColIndex,
-                                    rowSpan, colSpan,
-                                    actualRowSpan, actualColSpan,
-                                    isSelected);
+  rv = tableLayout->GetCellDataAt(aRowIndex, aColIndex, aCell,
+                                  startRowIndex, startColIndex,
+                                  rowSpan, colSpan,
+                                  actualRowSpan, actualColSpan, isSelected);
+
+  if (rv == NS_TABLELAYOUT_CELL_NOT_FOUND)
+    return NS_ERROR_INVALID_ARG;
+  return rv;
 }
 
 NS_IMETHODIMP nsHTMLTableAccessible::GetDescription(nsAString& aDescription)
@@ -1030,7 +1041,9 @@ NS_IMETHODIMP nsHTMLTableAccessible::IsProbablyForLayout(PRBool *aIsProbablyForL
   // Check to see if there are visible borders on the cells
   // XXX currently, we just check the first cell -- do we really need to do more?
   nsCOMPtr<nsIDOMElement> cellElement;
-  GetCellAt(0, 0, *getter_AddRefs(cellElement));
+  nsresult rv = GetCellAt(0, 0, *getter_AddRefs(cellElement));
+  NS_ENSURE_SUCCESS(rv, NS_ERROR_FAILURE);
+
   nsCOMPtr<nsIContent> cellContent(do_QueryInterface(cellElement));
   NS_ENSURE_TRUE(cellContent, NS_ERROR_FAILURE);
   nsCOMPtr<nsIPresShell> shell(GetPresShell());
diff --git a/mozilla/accessible/tests/mochitest/test_nsIAccessibleTable_2.html b/mozilla/accessible/tests/mochitest/test_nsIAccessibleTable_2.html
index f7b3b195634..b6642f72f9b 100644
--- a/mozilla/accessible/tests/mochitest/test_nsIAccessibleTable_2.html
+++ b/mozilla/accessible/tests/mochitest/test_nsIAccessibleTable_2.html
@@ -12,10 +12,11 @@ function doTest()
 {
   netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
 
-  var table = document.getElementById("table");
   var accService = Components.classes["@mozilla.org/accessibleRetrieval;1"].
                    getService(Components.interfaces.nsIAccessibleRetrieval);
 
+  // bug 410052
+  var table = document.getElementById("table");
   var accTable = accService.getAccessibleFor(table).
                  QueryInterface(Components.interfaces.nsIAccessibleTable);
 
@@ -29,15 +30,45 @@ function doTest()
   is(accTable.getRowExtentAt(2,7), 4,"rowspan wrong");
   is(accTable.getColumnExtentAt(2,3), 1, "colspan wrong");
   is(accTable.cellRefAt(2,1).firstChild.name, "c1", "wrong cell");
+
+  // bug 417912
+  var table2 = document.getElementById("table2");
+  var accTable2 = accService.getAccessibleFor(table2).
+                  QueryInterface(Components.interfaces.nsIAccessibleTable);
+  testCellAt(accTable2, 0, 0, true);
+  testCellAt(accTable2, 0, 1, true);
+  testCellAt(accTable2, 0, 2, true);
+  testCellAt(accTable2, 1, 0, true);
+  testCellAt(accTable2, 1, 1, false);
+  testCellAt(accTable2, 1, 2, true);
+  testCellAt(accTable2, 2, 0, true);
+  testCellAt(accTable2, 2, 1, true);
+  testCellAt(accTable2, 2, 2, true);
+
   SimpleTest.finish();
 }
 
+function testCellAt(aTable, aRow, aColumn, aSucceeded)
+{
+  try {
+    aTable.cellRefAt(aRow, aColumn);
+    ok(aSucceeded, "cell is available at (" + aRow + ", " + aColumn + ").");
+  } catch (e) {
+    ok(!aSucceeded, "cell is not available at (" + aRow + ", " + aColumn + ").");
+  }
+}
+
 SimpleTest.waitForExplicitFinish();
 addLoadEvent(doTest);
   </script>
  </head>
  <body>
   <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=410052">Mozilla Bug 410052</a>
+  <br>
+  <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=417912"
+                     title="GetCellDataAt callers that expect an error if no cell is found are wrong">
+    Mozilla Bug 417912
+  </a>
   <p id="display"></p>
   <div id="content" style="display: none"></div>
   <pre id="test">
@@ -83,5 +114,14 @@ addLoadEvent(doTest);
     </tbody>
    </table>
   </center>
+
+  <br><br><b>Testing Table 2:</b><br><br>
+  <center>
+    <table cellpadding="2" cellspacing="2" border="1" width="50%" id="table2">
+      <tr><td>1</td><td>2</td><td rowspan=3>3</td>
+      <tr><td>4</td>
+      <tr><td>5</td><td>6</td>
+    </table>
+  </center>
  </body>
 </html>