Stack challenge

stackChallenge.txt

A stack machine processes instructions by pushing and popping values to an internal stack. A simple example of this is a calculator.

The argument passed to run(instructions) will always be a string containing a series of instructions.
The instruction set of the calculator will be this:

+: Pop the last 2 values from the stack, add them, and push the result onto the stack.
-: Pop the last 2 values from the stack, subtract the lower one from the topmost one, and push the result.
*: Pop the last 2 values, multiply, and push the result.
/: Pop the last 2 values, divide the topmost one by the lower one, and push the result.
DUP: Duplicate (not double) the top value on the stack.
POP: Pop the last value from the stack and discard it.
PSH: Performed whenever a number appears as an instruction. Push the number to the stack.
Any other instruction (for example, a letter) should result in the value "Invalid instruction: [instruction]"
Examples
StackCalc("") ➞ 0

StackCalc("5 6 +") ➞ 11

StackCalc("3 DUP +") ➞ 6

StackCalc("6 5 5 7 * - /") ➞ 5

StackCalc("x y +") ➞ Invalid instruction: x