diff --git a/lib/fast486/common.inl b/lib/fast486/common.inl
index 895487ca646..57b2df7ef87 100644
--- a/lib/fast486/common.inl
+++ b/lib/fast486/common.inl
@@ -159,7 +159,7 @@ Fast486ReadLinearMemory(PFAST486_STATE State,
             if (Page == PAGE_ALIGN(LinearAddress + Size - 1))
             {
                 /* Copy only a part of the page */
-                PageLength = PAGE_OFFSET(LinearAddress + Size) - PageOffset;
+                PageLength = PAGE_OFFSET(LinearAddress) + Size - PageOffset;
             }
 
             /* Read the memory */
@@ -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) - PageOffset;
+                PageLength = PAGE_OFFSET(LinearAddress) + Size - PageOffset;
             }
 
             /* Write the memory */