diff --git a/windows/winproc.c b/windows/winproc.c
index d07720614bb..1bb9bb99962 100644
--- a/windows/winproc.c
+++ b/windows/winproc.c
@@ -594,13 +594,13 @@ INT WINPROC_MapMsg32ATo32W( HWND hwnd, UINT msg, WPARAM *pwparam, LPARAM *plpara
     case LB_DIR:
     case LB_ADDFILE:
     case EM_REPLACESEL:
-    {
-        UNICODE_STRING usBuffer;
-        if(!*plparam) return 0;
-        RtlCreateUnicodeStringFromAsciiz(&usBuffer,(LPCSTR)*plparam);
-        *plparam = (LPARAM)usBuffer.Buffer;
-        return (*plparam ? 1 : -1);
-    }
+        {
+            DWORD len = MultiByteToWideChar(CP_ACP, 0, (LPCSTR)*plparam, -1, NULL, 0);
+            WCHAR *buf = HeapAlloc(GetProcessHeap(), 0, len * sizeof(WCHAR));
+            len = MultiByteToWideChar(CP_ACP, 0, (LPCSTR)*plparam, -1, buf, len);
+            *plparam = (LPARAM)buf;
+            return (*plparam ? 1 : -1);
+        }
     case WM_GETTEXTLENGTH:
     case CB_GETLBTEXTLEN:
     case LB_GETTEXTLEN: