diff --git a/src/libsystemd/sd-journal/mmap-cache.c b/src/libsystemd/sd-journal/mmap-cache.c
index 6c2c26fde8..992bf1c4fc 100644
--- a/src/libsystemd/sd-journal/mmap-cache.c
+++ b/src/libsystemd/sd-journal/mmap-cache.c
@@ -551,6 +551,8 @@ int mmap_cache_add_fd(MMapCache *m, int fd, int prot, MMapFileDescriptor **ret)
 
         existing = hashmap_get(m->fds, FD_TO_PTR(fd));
         if (existing) {
+                if (existing->prot != prot)
+                        return -EEXIST;
                 if (ret)
                         *ret = existing;
                 return 0;