diff --git a/lib/diskpart.c b/lib/diskpart.c
index 999d8ed..06f27ac 100644
--- a/lib/diskpart.c
+++ b/lib/diskpart.c
@@ -68,11 +68,11 @@
 		break;
 	    }
 
-	    mptab[extnum].start = __le32_to_cpu(ptab[0].start_sect) + offset;
-	    mptab[extnum].length = __le32_to_cpu(ptab[0].nr_sects);
+	    mptab[extnum].start = offset + (__u32)__le32_to_cpu(ptab[0].start_sect);
+	    mptab[extnum].length = (__u32) __le32_to_cpu(ptab[0].nr_sects);
 	    mptab[extnum].id = ptab[0].sys_ind;
 
-	    offset += __le32_to_cpu(ptab[1].start_sect);
+	    offset = mptab[ex].start + (__u32) __le32_to_cpu(ptab[1].start_sect);
 
 	    extnum++;
 	    if (!is_extended(ptab[1].sys_ind))