diff --git a/WinDivertSharp/WinDivert.cs b/WinDivertSharp/WinDivert.cs
index ac368fb..2c444d2 100644
--- a/WinDivertSharp/WinDivert.cs
+++ b/WinDivertSharp/WinDivert.cs
@@ -610,7 +610,7 @@ public static bool WinDivertHelperCheckFilter(string filter, WinDivertLayer laye
             char* pErrorString = null;
 
             uint errPosTmp = 0;
-            var retVal = WinDivertNative.WinDivertHelperCheckFilter(filter, layer, &pErrorString, ref errorPosition);
+            var retVal = WinDivertNative.WinDivertHelperCheckFilter(filter, layer, &pErrorString, ref errPosTmp);
             if (pErrorString != null)
             {
                 errorMessage = Marshal.PtrToStringAnsi((IntPtr)pErrorString);
@@ -661,4 +661,4 @@ public static bool WinDivertHelperEvalFilter(string filter, WinDivertLayer layer
             }
         }
     }
-}
\ No newline at end of file
+}