wrong offset addition for store 64B

Hello,
I have implemented store instruction for 16xi32 64B. it works finely except that it has wrong offset addition. the offset should be 64 but here i m getting it 4.

Legalizing: t277: ch = store<ST64FixedStack7(align=64)+4> t0, t322, t276, undef:i64

Legalizing: t276: i64 = add FrameIndex:i64<7>, Constant:i64<4>

here it should be
Legalizing: t276: i64 = add FrameIndex:i64<7>, Constant:i64<64>

Why is that so?

Where am I doing mistake? otherwise store 128B works fine in this code of matrix multiplication.

Please help.