diff --git a/lib/fast486/common.inl b/lib/fast486/common.inl
index 6fb0398e3dd..895487ca646 100644
--- a/lib/fast486/common.inl
+++ b/lib/fast486/common.inl
@@ -224,7 +224,7 @@ Fast486WriteLinearMemory(PFAST486_STATE State,
             if (Page == PAGE_ALIGN(LinearAddress + Size - 1))
             {
                 /* Copy only a part of the page */
-                PageLength = PAGE_OFFSET(LinearAddress + Size);
+                PageLength = PAGE_OFFSET(LinearAddress + Size) - PageOffset;
             }
 
             /* Write the memory */