diff --git a/Source/cmAffinity.cxx b/Source/cmAffinity.cxx
index 588b2f2f52..09b029866d 100644
--- a/Source/cmAffinity.cxx
+++ b/Source/cmAffinity.cxx
@@ -12,6 +12,8 @@
 #    define CM_HAVE_CPU_AFFINITY
 #    include <pthread.h>
 #    include <sched.h>
+// On some platforms CPU_ZERO needs memset but sched.h forgets string.h
+#    include <string.h> // IWYU pragma: keep
 #    if defined(__FreeBSD__)
 #      include <pthread_np.h>
 #      include <sys/cpuset.h>