diff --git a/dlls/quartz/main.c b/dlls/quartz/main.c
index 08287a75ebc..282104e19c5 100644
--- a/dlls/quartz/main.c
+++ b/dlls/quartz/main.c
@@ -313,7 +313,13 @@ DWORD WINAPI AMGetErrorTextA(HRESULT hr, LPSTR buffer, DWORD maxlen)
         return 0;
 
     res = AMGetErrorTextW(hr, errorW, ARRAY_SIZE(errorW));
-    return WideCharToMultiByte(CP_ACP, 0, errorW, res, buffer, maxlen, 0, 0);
+    if (!res)
+        return 0;
+
+    res = WideCharToMultiByte(CP_ACP, 0, errorW, -1, NULL, 0, 0, 0);
+    if (res > maxlen || !res)
+        return 0;
+    return WideCharToMultiByte(CP_ACP, 0, errorW, -1, buffer, maxlen, 0, 0) - 1;
 }
 
 /***********************************************************************