src_suffix getings ignored

Greetings,

I have the following builder:
import os

template_suffix = ".temp"

def convert_path(base, out, src):
    return src.replace(base, out)

def modify_target(target, source, env):
    return convert_path(env['base'], env['out_dir'], str(source[0])), source

def a_action(target, source, action):
    return os.system("echo -" + action + " " + os.path.basename(source))

def a_b_action(target, source, env):
    return a_action(target, source, "a")

def a_c_action(target, source, env):
    return a_action(target, source, "b")

def generate(env, **kwargs):
    builders = {
        'do_a_b' : env.Builder(action = env.Action(a_b_action), suffix = '.c',
                               src_suffix = template_suffix, emitter = modify_target),
        'do_a_c' : env.Builder(action = env.Action(a_c_action), suffix = '.h',
                               src_suffix = template_suffix, emitter = modify_target)
    }

    env.Append(BUILDERS = builders)

def exists(env):
    return true

when I add prints to modify_target, both target and source looks good.
however, when I run it as part of a scons build flow, it fails with the following error:
scons --tree=all --debug=explain -n -Q
scons: *** While building `['/tmp/proj/output/objs/module_a/generated/dir1/file.o']' from `['generated/dir1/file.temp']': Don't know how to build from a source file with suffix `.temp'.  Expected a suffix in this list: ['.c', '.m', '.cpp', '.cc', '.cxx', '.c++', '.C++', '.mm', '.C', '.f', '.for', '.ftn', '.fpp', '.FPP', '.F', '.FOR', '.FTN', '.f77', '.F77', '.f90', '.F90', '.f95', '.F95', '.f03', '.F03', '.f08', '.F08', '.s', '.asm', '.ASM', '.spp', '.SPP', '.sx', '.S', '.d'].

any ideas what I'm doing wrong?

Thanks.