--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -2020,18 +2020,21 @@ def getJSToNativeConversionTemplate(type
     assert not (isEnforceRange and isClamp) # These are mutually exclusive
 
     if type.isArray():
         raise TypeError("Can't handle array arguments yet")
 
     if type.isSequence():
         assert not isEnforceRange and not isClamp
 
-        if failureCode is not None:
-            raise TypeError("Can't handle sequences when failureCode is not None")
+        if failureCode is None:
+            notSequence = "return ThrowErrorMessage(cx, MSG_NOT_SEQUENCE);"
+        else:
+            notSequence = failureCode
+
         nullable = type.nullable();
         # Be very careful not to change "type": we need it later
         if nullable:
             elementType = type.inner.inner
         else:
             elementType = type.inner
 
         # We have to be careful with reallocation behavior for arrays.  In
@@ -2066,34 +2069,35 @@ def getJSToNativeConversionTemplate(type
             arrayRef = "${declName}"
         # If we're optional, the const will come from the Optional
         mutableTypeName = typeName
         if not isOptional:
             typeName = CGWrapper(typeName, pre="const ")
 
         templateBody = ("""JSObject* seq = &${val}.toObject();\n
 if (!IsArrayLike(cx, seq)) {
-  return ThrowErrorMessage(cx, MSG_NOT_SEQUENCE);
+%s
 }
 uint32_t length;
 // JS_GetArrayLength actually works on all objects
 if (!JS_GetArrayLength(cx, seq, &length)) {
   return false;
 }
 Sequence< %s > &arr = const_cast< Sequence< %s >& >(%s);
 if (!arr.SetCapacity(length)) {
   JS_ReportOutOfMemory(cx);
   return false;
 }
 for (uint32_t i = 0; i < length; ++i) {
   jsval temp;
   if (!JS_GetElement(cx, seq, i, &temp)) {
     return false;
   }
-""" % (elementDeclType.define(),
+""" % (CGIndenter(CGGeneric(notSequence)).define(),
+       elementDeclType.define(),
        elementDeclType.define(),
        arrayRef))
 
         templateBody += CGIndenter(CGGeneric(
                 string.Template(elementTemplate).substitute(
                     {
                         "val" : "temp",
                         "valPtr": "&temp",