Error in processing Complex arguments in trig functions

Folks,

   Here's the problem:
CL-USER 1 > (acos #c(2 +0.0))

Error: Arithmetic error in COMPLEX of (#C(0.0D0 0.7071067811865476D0)): 
Arguments must be real numbers.
   1 (continue) Return a value to use.
   2 (abort) Return to level 0.
   3 Return to top loop level 0.

Type :b for backtrace, :c <option number> to proceed,  or :? for other 
options

CL-USER 2 : 1 :c3
:C3

CL-USER 3 : 1 > :c 3

CL-USER 4 > (acos #c(2 0))
#C(0.0 1.316958)

CL-USER 5 > (acos #c(2.0 0))

Error: Arithmetic error in COMPLEX of (#C(0.0D0 0.7071067811865476D0)): 
Arguments must be real numbers.
   1 (continue) Return a value to use.
   2 (abort) Return to level 0.
   3 Return to top loop level 0.

Type :b for backtrace, :c <option number> to proceed,  or :? for other 
options

CL-USER 6 : 1 > :c 3

CL-USER 7 > (acos #c(2 0.0))

Error: Arithmetic error in COMPLEX of (#C(0.0D0 0.7071067811865476D0)): 
Arguments must be real numbers.
   1 (continue) Return a value to use.
   2 (abort) Return to level 0.
   3 Return to top loop level 0.

Type :b for backtrace, :c <option number> to proceed,  or :? for other 
options

CL-USER 8 : 1 > :c 3

CL-USER 9 > (acos #c(2.0 0.0))

Error: Arithmetic error in COMPLEX of (#C(0.0D0 0.7071067811865476D0)): 
Arguments must be real numbers.
   1 (continue) Return a value to use.
   2 (abort) Return to level 0.
   3 Return to top loop level 0.

Type :b for backtrace, :c <option number> to proceed,  or :? for other 
options

CL-USER 10 : 1 > :c 3

CL-USER 11 >