Warning in `parseIOManagerFlag`

The nightly-x86_64-linux-deb12-validate+thread_sanitizer_cmm job fails with the following warning:

rts/IOManager.c: In function ‘parseIOManagerFlag’:
rts/IOManager.c:177:1: error:
     error: control reaches end of non-void function [-Werror=return-type]
      177 | }
          | ^
    |
177 | }
    | ^

It's not immediately apparently what is going on here, but it deserves investigation.