diff --git a/ipc/mscom/Utils.cpp b/ipc/mscom/Utils.cpp
index ab6ac719696b..13ef4d55217d 100644
--- a/ipc/mscom/Utils.cpp
+++ b/ipc/mscom/Utils.cpp
@@ -128,8 +128,8 @@ uintptr_t GetContainingModuleHandle() {
   return reinterpret_cast<uintptr_t>(thisModule);
 }
 
-uint32_t CreateStream(const uint8_t* aInitBuf, const uint32_t aInitBufSize,
-                      IStream** aOutStream) {
+long CreateStream(const uint8_t* aInitBuf, const uint32_t aInitBufSize,
+                  IStream** aOutStream) {
   if (!aInitBufSize || !aOutStream) {
     return E_INVALIDARG;
   }
@@ -208,7 +208,7 @@ uint32_t CreateStream(const uint8_t* aInitBuf, const uint32_t aInitBufSize,
   return S_OK;
 }
 
-uint32_t CopySerializedProxy(IStream* aInStream, IStream** aOutStream) {
+long CopySerializedProxy(IStream* aInStream, IStream** aOutStream) {
   if (!aInStream || !aOutStream) {
     return E_INVALIDARG;
   }
diff --git a/ipc/mscom/Utils.h b/ipc/mscom/Utils.h
index d93740fe7425..63011dc1df7a 100644
--- a/ipc/mscom/Utils.h
+++ b/ipc/mscom/Utils.h
@@ -38,8 +38,8 @@ uintptr_t GetContainingModuleHandle();
  * @param aOutStream Outparam to receive the newly created stream.
  * @return HRESULT error code.
  */
-uint32_t CreateStream(const uint8_t* aBuf, const uint32_t aBufLen,
-                      IStream** aOutStream);
+long CreateStream(const uint8_t* aBuf, const uint32_t aBufLen,
+                  IStream** aOutStream);
 
 /**
  * Creates a deep copy of a proxy contained in a stream.
@@ -48,7 +48,7 @@ uint32_t CreateStream(const uint8_t* aBuf, const uint32_t aBufLen,
  * @param aOutStream Outparam to receive the newly created stream.
  * @return HRESULT error code.
  */
-uint32_t CopySerializedProxy(IStream* aInStream, IStream** aOutStream);
+long CopySerializedProxy(IStream* aInStream, IStream** aOutStream);
 
 #if defined(MOZILLA_INTERNAL_API)
 /**