diff --git a/js/ctypes/Function.cpp b/js/ctypes/Function.cpp
index 75e5b0866d2b..bb256a02a66c 100644
--- a/js/ctypes/Function.cpp
+++ b/js/ctypes/Function.cpp
@@ -163,7 +163,7 @@ GetABI(PRUint16 aCallType, ffi_abi& aResult)
   case nsIForeignLibrary::DEFAULT:
     aResult = FFI_DEFAULT_ABI;
     return true;
-#if defined(_WIN32)
+#if defined(_WIN32) && !defined(_WIN64)
   case nsIForeignLibrary::STDCALL:
     aResult = FFI_STDCALL;
     return true;