Why doesn't the assembler treat a macro as an immediate?

I’m trying to print a string through a system call, use macros to refer to string length. When I assemble this code with clang, it complains that the macros ‘LEN’ must be a 32-bit immediate. The compilation command used is as follows:

clang --target=riscv32 -march=rv32imafcv -c -mno-relax hello.s -o hello.o

The assembler source code is shown below:

        .option norelax
        .section .rodata
        .align  2
msg:    .ascii  "hello world!\n"
        .equ    LEN, .-msg
        .type   msg, object
        .size   msg, LEN

	    .globl _start
	    .align 4
        li      a0, 1
        la      a1, msg
        li      a2, LEN
        li      a7, 64    /* write */
	    li	a0, 0
	    li	a7, 93

It seems to me that the ‘LEN’ should be an immediate with the value 13, but clang doesn’t seem to think so:

hello.s:14:21: error: immediate must be an integer in the range [-2147483648, 4294967295]
        li      a2, LEN

Furthermore, If I change this line as follows,

        li      a2, 13

the program works:

$ld.lld --no-relax hello.o -o hello
$qemu-riscv32 hello
hello world!

I don’t know why the macro ‘LEN’ can’t be identified correctly by clang, and the clang version is 14.0.6. Any suggestions would be helpful, thanks!