page fault error with SIMD strlen (using SWAR in integer registers, not SSE)

Question

The asm function strlen receives the link to an string as a char - Array. To to so, the functions may use SIMD on general purpose register, but without using xmm register or SSE instructions.

The functions checks with bit manipulation in 8 Byte steps, if the string contains the zero which marks the end. If it does, it checks byte per byte if it is zero. It works but I get page fault exceptions, which I don't know how to solve since the functions iterates in single steps in the last loop at the end.

        ; rdi <- const *char
        ; rax <- counter + return value
        ; rbx <- current array
        ; rcx <- Bitmask
        ; rsi <- Arr for calculation
       
    
    strlen:
        PUSH rbx
        PUSH rbp
        MOV  rbp,rsp
        SUB rsp,8      ; calling convention
    
        XOR rax,rax
        MOV rcx,0x7F7F7F7F7F7F7F7F
    
    while_parallel: ;checks if next 8 byte contains end - zero 
        MOV rbx,[rdi+rax]
        MOV rsi,rbx
    
        AND rsi,rcx
        ADD rsi,rcx
        OR  rsi,rbx
        OR  rsi,rcx
        NOT rsi
    
        CMP rsi, qword 0
        JNE while_single
        ADD rax,8
        JMP while_parallel
    
    while_single:  ;check if next byte is zero
        CMP [rdi+rax],byte 0
        JE  end
        INC rax
        JMP while_single
   end:
    ADD rsp,8
    POP rbp
    POP rbx

   RET

Note that I must not use any SSE instructions nor xmm register.