sandbox egg: unusual build failure when trying to port to CHICKEN 6

I'm trying to get the sandbox egg to build under CHICKEN 6. Replacing the scheme import with (scheme base) (scheme cxr) (scheme write) makes it compile to C with no errors, but it fails during C compilation:
$ /opt/chicken-6.0.0pre2/bin/chicken-install -n
building sandbox
  /opt/chicken-6.0.0pre2/bin/csc -host -D compiling-extension -J -s -regenerate-import-libraries -setup-mode -I /home/caelorum/.cache/chicken-install/sandbox -C -I/home/caelorum/.cache/chicken-install/sandbox -O3 -d1 sandbox.scm -o /home/caelorum/.cache/chicken-install/sandbox/sandbox.so
/home/caelorum/.cache/chicken-install/sandbox/sandbox.c:14375:49: error: 
     call to undeclared function 'C_substring_compare'; ISO C99 and later do
     not support implicit function declarations
     [-Wimplicit-function-declaration]
14375 | av2[1]=(C_truep(C_byteblockp(((C_word*)t0)[3]))?C_substring_compare(((C...
      |                                                 ^
1 error generated.
Error: shell command terminated with non-zero exit status 1: 'clang' '/home/caelorum/.cache/chicken-install/sandbox/sandbox.c' '-o' '/home/caelorum/.cache/chicken-install/sandbox/sandbox.o' '-c' '-fno-strict-aliasing' '-fwrapv' '-DHAVE_CHICKEN_CONFIG_H' '-DC_ENABLE_PTABLES' '-Os' '-fomit-frame-pointer' '-fPIC' '-DPIC' '-DC_SHARED' '-I/home/caelorum/.cache/chicken-install/sandbox' '-I/opt/chicken-6.0.0pre2/include/chicken'
Error: shell command terminated with nonzero exit code
256
"sh /home/caelorum/.cache/chicken-install/sandbox/sandbox.build.sh"