"unsupported initializer for address space" in Opencl on Linux Mint - opencl

Question

I get the following Error when i try to run the following pyopencl code on my linxu mint computer with a Radeon grafic card.
unsupported initializer for address space in wordpuzzle
The program is easy to understand: I have a INT Array which i copy to kernel. I read out the first number of the INT Array and based on the returned number i want to pick the char from the CharSSet and return it.
When i replace
wordbuff[0] = charSSet[a_g[0]];
with
wordbuff[0] = charSSet[10];
then it works well!
WHY???
el Code:
import numpy as np
import pyopencl as cl
a_np = np.array([10,22,5,3,8])
ctx = cl.create_some_context()
queue = cl.CommandQueue(ctx)
mf = cl.mem_flags
a_g = cl.Buffer(ctx, mf.READ_ONLY | mf.COPY_HOST_PTR, hostbuf=a_np)
wordValue = np.array(['\0','\0','\0','\0','\0','\0'])
word_buffer = cl.Buffer(ctx, mf.READ_WRITE | mf.COPY_HOST_PTR, hostbuf=wordValue)
prg = cl.Program(ctx, """
__kernel void wordpuzzle(
__global const int *a_g, __global unsigned char *wordbuff)
{
int gid = get_global_id(0);
char charSSet[53] = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
if (gid == 0){
wordbuff[0] = charSSet[a_g[0]];
}
}
""").build()
res_g = cl.Buffer(ctx, mf.WRITE_ONLY, a_np.nbytes)
prg.wordpuzzle(queue, (1,), (1,), a_g, word_buffer)
theChars = np.empty_like(wordValue)
cl.enqueue_read_buffer(queue,word_buffer,theChars)
print(theChars)
THE WHOLE ERROR REPORT:
Choose platform: [0]
Choice [0]: Set the environment variable PYOPENCL_CTX='' to avoid
being asked again. Traceback (most recent call last): File
"demo.py", line 35, in
""").build() File "/usr/lib/python2.7/dist-packages/pyopencl/init.py", line 213, in
build
options=options, source=self._source) File "/usr/lib/python2.7/dist-packages/pyopencl/init.py", line 253, in
_build_and_catch_errors
raise err pyopencl.RuntimeError: clBuildProgram failed: build program failure -
Build on :
unsupported initializer for address space in wordpuzzle (options: -I
/usr/lib/python2.7/dist-packages/pyopencl/cl) (source saved as
/tmp/tmp7PxezZ.cl)