diff --git a/otherlibs/systhreads/Makefile b/otherlibs/systhreads/Makefile
index cf4b8061a..09a6b7889 100644
--- a/otherlibs/systhreads/Makefile
+++ b/otherlibs/systhreads/Makefile
@@ -27,7 +27,7 @@ libthreadsnat.a: $(NATIVECODE_C_OBJS)
 	ar rc libthreadsnat.a $(NATIVECODE_C_OBJS)
 
 posix_n.o: posix.c
-	$(NATIVECC) -O -I ../../asmrun -I../../byterun $(NATIVECCCOMPOPTS) -DNATIVE_CODE -DTARGET_$(ARCH) -DSYS_$(SYSTEM) -c posix.c
+	$(NATIVECC) -g -I../../asmrun -I../../byterun $(NATIVECCCOMPOPTS) -DNATIVE_CODE -DTARGET_$(ARCH) -DSYS_$(SYSTEM) -c posix.c
 	mv posix.o posix_n.o
 
 threads.cma: $(THREAD_OBJS)
diff --git a/otherlibs/systhreads/posix.c b/otherlibs/systhreads/posix.c
index f7b307642..cce502932 100644
--- a/otherlibs/systhreads/posix.c
+++ b/otherlibs/systhreads/posix.c
@@ -251,6 +251,7 @@ static void * caml_thread_tick(void * arg)
     something_to_do = 1;
 #endif
   }
+  return NULL;                  /* prevents compiler warning */
 }
 
 /* Thread cleanup: remove the descriptor from the list and free