diff --git a/intl/uconv/util/nsUCSupport.cpp b/intl/uconv/util/nsUCSupport.cpp
index b25fb9e27dbd..ba7125e3e2bd 100644
--- a/intl/uconv/util/nsUCSupport.cpp
+++ b/intl/uconv/util/nsUCSupport.cpp
@@ -66,7 +66,11 @@ nsBasicDecoderSupport::~nsBasicDecoderSupport()
 
 NS_IMPL_ADDREF(nsBasicDecoderSupport);
 NS_IMPL_RELEASE(nsBasicDecoderSupport);
+#ifdef NS_DEBUG
+NS_IMPL_QUERY_INTERFACE2(nsBasicDecoderSupport, nsIUnicodeDecoder, nsIBasicDecoder);
+#else
 NS_IMPL_QUERY_INTERFACE1(nsBasicDecoderSupport, nsIUnicodeDecoder)
+#endif
 
 //----------------------------------------------------------------------
 // Interface nsIUnicodeDecoder [implementation]
@@ -366,10 +370,15 @@ nsBasicEncoder::~nsBasicEncoder()
 
 NS_IMPL_ADDREF(nsBasicEncoder);
 NS_IMPL_RELEASE(nsBasicEncoder);
+#ifdef NS_DEBUG
+NS_IMPL_QUERY_INTERFACE3(nsBasicEncoder,
+                         nsIUnicodeEncoder,
+                         nsICharRepresentable, nsIBasicEncoder)
+#else
 NS_IMPL_QUERY_INTERFACE2(nsBasicEncoder,
                          nsIUnicodeEncoder,
                          nsICharRepresentable)
-
+#endif
 //----------------------------------------------------------------------
 // Class nsEncoderSupport [implementation]
 
diff --git a/intl/uconv/util/nsUCSupport.h b/intl/uconv/util/nsUCSupport.h
index b83a50fb79e4..2e26f7779c39 100644
--- a/intl/uconv/util/nsUCSupport.h
+++ b/intl/uconv/util/nsUCSupport.h
@@ -47,6 +47,27 @@
 
 #define ONE_BYTE_TABLE_SIZE 256
 
+#ifdef NS_DEBUG
+// {7AFC9F0A-CFE1-44ea-A755-E3B86AB1226E}
+#define NS_IBASICDECODER_IID \
+{ 0x7afc9f0a, 0xcfe1, 0x44ea, { 0xa7, 0x55, 0xe3, 0xb8, 0x6a, 0xb1, 0x22, 0x6e } }
+
+// {65968A7B-6467-4c4a-B50A-3E0C97A32F07}
+#define NS_IBASICENCODER_IID \
+{ 0x65968a7b, 0x6467, 0x4c4a, { 0xb5, 0xa, 0x3e, 0xc, 0x97, 0xa3, 0x2f, 0x7 } }
+
+class nsIBasicDecoder : public nsISupports {
+public:
+  NS_DEFINE_STATIC_IID_ACCESSOR(NS_IBASICDECODER_IID);
+};
+
+class nsIBasicEncoder : public nsISupports {
+public:
+  NS_DEFINE_STATIC_IID_ACCESSOR(NS_IBASICENCODER_IID);
+};
+
+#endif
+
 //----------------------------------------------------------------------
 // Class nsBasicDecoderSupport [declaration]
 
@@ -66,6 +87,9 @@
  * @author  Catalin Rotaru [CATA]
  */
 class nsBasicDecoderSupport : public nsIUnicodeDecoder
+#ifdef NS_DEBUG
+                              ,public nsIBasicDecoder
+#endif
 {
   NS_DECL_ISUPPORTS
 
@@ -265,6 +289,9 @@ protected:
 // Class nsBasicEncoder [declaration]
 
 class nsBasicEncoder : public nsIUnicodeEncoder, public nsICharRepresentable
+#ifdef NS_DEBUG
+                       ,public nsIBasicEncoder
+#endif
 {
   NS_DECL_ISUPPORTS