IMPRECISERR error when decrementing Stack Pointer

Hello,

I am using a STM32F4 series board with Arm Cortex M4 processor to develop a simple scheduler for learning purposes. I have a code snippet where I first set up the stack start location for a task, and then initialize PSP (stack pointer) to its very first location, and then manually fill the stack with some values as follows:

In the above code when I execute the very first "pTask--;", I get an  IMPRECISERR. However when I modify the sequence as below, the exception went away

//Modified Code Start

pTask = (uint32_t *)TASK1_STACK_START; //Stack Start location for Task1

pTask--; //Decrement

*pTask = (uint32_t) SET_XPSR_FOR_THUMB; //Store the xPSR value in address :TASK1_STACK_START - 4

 //Modified code end 

Although I am aware that the decrement needs to happen before setting a value in Arm Cortex, I was setting the xPSR value first in order to make use of the very first Stack location TASK1_STACK_START.   (in the second case, the first stack location TASK1_STACK_START will go unused) 

Could someone tell me why this is the case ? Is there any internal check in Arm processors whereby decrements are mandated before storing value to a stack location , and throw an exception otherwise ? 

Thanks