--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -5,17 +5,17 @@
 # Common codegen classes.
 
 import os
 import re
 import string
 import math
 import textwrap
 
-from WebIDL import BuiltinTypes, IDLBuiltinType, IDLNullValue, IDLSequenceType, IDLType, IDLAttribute, IDLUndefinedValue
+from WebIDL import BuiltinTypes, IDLBuiltinType, IDLNullValue, IDLSequenceType, IDLType, IDLAttribute, IDLUndefinedValue, IDLEmptySequenceValue
 from Configuration import NoSuchDescriptorError, getTypesFromDescriptor, getTypesFromDictionary, getTypesFromCallback, Descriptor
 
 AUTOGENERATED_WARNING_COMMENT = \
     "/* THIS FILE IS AUTOGENERATED - DO NOT EDIT */\n\n"
 ADDPROPERTY_HOOK_NAME = '_addProperty'
 FINALIZE_HOOK_NAME = '_finalize'
 CONSTRUCT_HOOK_NAME = '_constructor'
 LEGACYCALLER_HOOK_NAME = '_legacycaller'
@@ -3574,24 +3574,16 @@ def getJSToNativeConversionInfo(type, de
             return template
         return (
             "if (${haveValue}) {\n" +
             indent(template) +
             "} else {\n" +
             indent(setDefault) +
             "}\n")
 
-    # A helper function for handling null default values.  Much like
-    # handleDefault, but checks that the default value, if it exists, is null.
-    def handleDefaultNull(template, codeToSetNull):
-        if (defaultValue is not None and
-            not isinstance(defaultValue, IDLNullValue)):
-            raise TypeError("Can't handle non-null default value here")
-        return handleDefault(template, codeToSetNull)
-
     # A helper function for wrapping up the template body for
     # possibly-nullable objecty stuff
     def wrapObjectTemplate(templateBody, type, codeToSetNull, failureCode=None):
         if isNullOrUndefined and type.nullable():
             # Just ignore templateBody and set ourselves to null.
             # Note that we don't have to worry about default values
             # here either, since we already examined this value.
             return codeToSetNull
@@ -3618,18 +3610,22 @@ def getJSToNativeConversionInfo(type, de
                   $*{failureBody}
                 }
                 """,
                 templateBody=templateBody,
                 elifLine=elifLine,
                 elifBody=elifBody,
                 failureBody=onFailureNotAnObject(failureCode).define())
 
-            if type.nullable():
-                templateBody = handleDefaultNull(templateBody, codeToSetNull)
+            if isinstance(defaultValue, IDLNullValue):
+                assert type.nullable() # Parser should enforce this
+                templateBody = handleDefault(templateBody, codeToSetNull)
+            elif isinstance(defaultValue, IDLEmptySequenceValue):
+                # Our caller will handle it
+                pass
             else:
                 assert defaultValue is None
 
         return templateBody
 
     # A helper function for converting things that look like a JSObject*.
     def handleJSObjectType(type, isMember, failureCode):
         if not isMember:
@@ -3758,16 +3754,23 @@ def getJSToNativeConversionInfo(type, de
             notSequence=notSequence,
             sequenceType=sequenceType,
             arrayRef=arrayRef,
             elementType=elementInfo.declType.define(),
             elementConversion=elementConversion)
 
         templateBody = wrapObjectTemplate(templateBody, type,
                                           "${declName}.SetNull();\n", notSequence)
+        if isinstance(defaultValue, IDLEmptySequenceValue):
+            if type.nullable():
+                codeToSetEmpty = "${declName}.SetValue();\n"
+            else:
+                codeToSetEmpty = "/* Array is already empty; nothing to do */\n"
+            templateBody = handleDefault(templateBody, codeToSetEmpty)
+
         # Sequence arguments that might contain traceable things need
         # to get traced
         if not isMember and typeNeedsRooting(elementType):
             holderType = CGTemplatedType("SequenceRooter", elementInfo.declType)
             # If our sequence is nullable, this will set the Nullable to be
             # not-null, but that's ok because we make an explicit SetNull() call
             # on it as needed if our JS value is actually null.
             holderArgs = "cx, &%s" % arrayRef

--- a/dom/bindings/test/TestBindingHeader.h
+++ b/dom/bindings/test/TestBindingHeader.h
@@ -374,18 +374,20 @@ public:
   void ReceiveWeakNullableCastableObjectSequence(nsTArray<TestInterface*> &);
   void ReceiveWeakCastableObjectNullableSequence(Nullable< nsTArray<TestInterface*> >&);
   void ReceiveWeakNullableCastableObjectNullableSequence(Nullable< nsTArray<TestInterface*> >&);
   void PassCastableObjectSequence(const Sequence< OwningNonNull<TestInterface> >&);
   void PassNullableCastableObjectSequence(const Sequence< nsRefPtr<TestInterface> > &);
   void PassCastableObjectNullableSequence(const Nullable< Sequence< OwningNonNull<TestInterface> > >&);
   void PassNullableCastableObjectNullableSequence(const Nullable< Sequence< nsRefPtr<TestInterface> > >&);
   void PassOptionalSequence(const Optional<Sequence<int32_t> >&);
+  void PassOptionalSequenceWithDefaultValue(const Sequence<int32_t> &);
   void PassOptionalNullableSequence(const Optional<Nullable<Sequence<int32_t> > >&);
   void PassOptionalNullableSequenceWithDefaultValue(const Nullable< Sequence<int32_t> >&);
+  void PassOptionalNullableSequenceWithDefaultValue2(const Nullable< Sequence<int32_t> >&);
   void PassOptionalObjectSequence(const Optional<Sequence<OwningNonNull<TestInterface> > >&);
   void PassExternalInterfaceSequence(const Sequence<nsRefPtr<TestExternalInterface> >&);
   void PassNullableExternalInterfaceSequence(const Sequence<nsRefPtr<TestExternalInterface> >&);
 
   void ReceiveStringSequence(nsTArray<nsString>&);
   void PassStringSequence(const Sequence<nsString>&);
 
   void ReceiveByteStringSequence(nsTArray<nsCString>&);

--- a/dom/bindings/test/TestCodeGen.webidl
+++ b/dom/bindings/test/TestCodeGen.webidl
@@ -335,18 +335,20 @@ interface TestInterface {
   sequence<TestInterface?> receiveWeakNullableCastableObjectSequence();
   sequence<TestInterface>? receiveWeakCastableObjectNullableSequence();
   sequence<TestInterface?>? receiveWeakNullableCastableObjectNullableSequence();
   void passCastableObjectSequence(sequence<TestInterface> arg);
   void passNullableCastableObjectSequence(sequence<TestInterface?> arg);
   void passCastableObjectNullableSequence(sequence<TestInterface>? arg);
   void passNullableCastableObjectNullableSequence(sequence<TestInterface?>? arg);
   void passOptionalSequence(optional sequence<long> arg);
+  void passOptionalSequenceWithDefaultValue(optional sequence<long> arg = []);
   void passOptionalNullableSequence(optional sequence<long>? arg);
   void passOptionalNullableSequenceWithDefaultValue(optional sequence<long>? arg = null);
+  void passOptionalNullableSequenceWithDefaultValue2(optional sequence<long>? arg = []);
   void passOptionalObjectSequence(optional sequence<TestInterface> arg);
   void passExternalInterfaceSequence(sequence<TestExternalInterface> arg);
   void passNullableExternalInterfaceSequence(sequence<TestExternalInterface?> arg);
 
   sequence<DOMString> receiveStringSequence();
   void passStringSequence(sequence<DOMString> arg);
 
   sequence<ByteString> receiveByteStringSequence();
@@ -877,16 +879,22 @@ dictionary Dict : ParentDict {
   (EventInit or long) eventInitOrLongWithDefaultValue3 = 5;
   (CustomEventInit or long) eventInitOrLongWithDefaultValue4 = 5;
 #endif
 
   ArrayBuffer arrayBuffer;
   ArrayBuffer? nullableArrayBuffer;
   Uint8Array uint8Array;
   Float64Array? float64Array = null;
+
+  sequence<long> seq1;
+  sequence<long> seq2 = [];
+  sequence<long>? seq3;
+  sequence<long>? seq4 = null;
+  sequence<long>? seq5 = [];
 };
 
 dictionary ParentDict : GrandparentDict {
   long c = 5;
   TestInterface someInterface;
   TestInterface? someNullableInterface = null;
   TestExternalInterface someExternalInterface;
   any parentAny;

--- a/dom/bindings/test/TestExampleGen.webidl
+++ b/dom/bindings/test/TestExampleGen.webidl
@@ -226,18 +226,20 @@ interface TestExampleInterface {
   sequence<TestInterface?> receiveWeakNullableCastableObjectSequence();
   sequence<TestInterface>? receiveWeakCastableObjectNullableSequence();
   sequence<TestInterface?>? receiveWeakNullableCastableObjectNullableSequence();
   void passCastableObjectSequence(sequence<TestInterface> arg);
   void passNullableCastableObjectSequence(sequence<TestInterface?> arg);
   void passCastableObjectNullableSequence(sequence<TestInterface>? arg);
   void passNullableCastableObjectNullableSequence(sequence<TestInterface?>? arg);
   void passOptionalSequence(optional sequence<long> arg);
+  void passOptionalSequenceWithDefaultValue(optional sequence<long> arg = []);
   void passOptionalNullableSequence(optional sequence<long>? arg);
   void passOptionalNullableSequenceWithDefaultValue(optional sequence<long>? arg = null);
+  void passOptionalNullableSequenceWithDefaultValue2(optional sequence<long>? arg = []);
   void passOptionalObjectSequence(optional sequence<TestInterface> arg);
   void passExternalInterfaceSequence(sequence<TestExternalInterface> arg);
   void passNullableExternalInterfaceSequence(sequence<TestExternalInterface?> arg);
 
   sequence<DOMString> receiveStringSequence();
   void passStringSequence(sequence<DOMString> arg);
 
   sequence<ByteString> receiveByteStringSequence();

--- a/dom/bindings/test/TestJSImplGen.webidl
+++ b/dom/bindings/test/TestJSImplGen.webidl
@@ -247,18 +247,20 @@ interface TestJSImplInterface {
   sequence<TestJSImplInterface?> receiveWeakNullableCastableObjectSequence();
   sequence<TestJSImplInterface>? receiveWeakCastableObjectNullableSequence();
   sequence<TestJSImplInterface?>? receiveWeakNullableCastableObjectNullableSequence();
   void passCastableObjectSequence(sequence<TestJSImplInterface> arg);
   void passNullableCastableObjectSequence(sequence<TestJSImplInterface?> arg);
   void passCastableObjectNullableSequence(sequence<TestJSImplInterface>? arg);
   void passNullableCastableObjectNullableSequence(sequence<TestJSImplInterface?>? arg);
   void passOptionalSequence(optional sequence<long> arg);
+  void passOptionalSequenceWithDefaultValue(optional sequence<long> arg = []);
   void passOptionalNullableSequence(optional sequence<long>? arg);
   void passOptionalNullableSequenceWithDefaultValue(optional sequence<long>? arg = null);
+  void passOptionalNullableSequenceWithDefaultValue2(optional sequence<long>? arg = []);
   void passOptionalObjectSequence(optional sequence<TestJSImplInterface> arg);
   void passExternalInterfaceSequence(sequence<TestExternalInterface> arg);
   void passNullableExternalInterfaceSequence(sequence<TestExternalInterface?> arg);
 
   sequence<DOMString> receiveStringSequence();
   sequence<ByteString> receiveByteStringSequence();
   // Callback interface problem.  See bug 843261.
   //void passStringSequence(sequence<DOMString> arg);