Can't write on the control register during handler mode

I am using TM4C development board and Uvision 4 IDE. During the context switch routine i need to store PSP value. When i use PSP symbol in the assembly code i have an error "error: A1647E: Bad register name symbol, expected Integer register" but this is not the case when i use SP only (without define explicitly MSP or PSP). 

I try to access the CONTROL register and write 2 to switch the stack pointer to PSP (and then i SP which i can use the assembly code will be the PSP) but the control register is not get changed. Could any one help?

Here is my code. 

Switch_Context                 ; 1) Saves R0-R3,R12,LR,PC,PSR 
    CPSID   I                  ; 2) Prevent interrupt during switch
    PUSH    {R4-R11}           ; 3) Save remaining regs r4-11
    LDR     R0, =RunPt         ; 4) R0=pointer to RunPt, old thread
    LDR     R1, [R0]           ; R1 = RunPt

    MOV     R2, #2             ; 
    MSR     CONTROL, R2 
    STR     SP, [R1]           ; 5) Save SP into TCB
    MOV     R2, #0             ; 
    MSR     CONTROL, R2
    .
    .
    .
    BX      LR                 ; 10) restore R0-R3,R12,LR,PC,PSR