diff --git a/src/ckpool.c b/src/ckpool.c
index 6e2f92d1..e74aa0ca 100644
--- a/src/ckpool.c
+++ b/src/ckpool.c
@@ -745,6 +745,8 @@ json_t *json_rpc_call(connsock_t *cs, const char *rpc_req)
 	double elapsed;
 	int len, ret;
 
+	/* Serialise all calls in case we use cs from multiple threads */
+	cksem_wait(&cs->sem);
 	if (unlikely(cs->fd < 0)) {
 		LOGWARNING("FD %d invalid in %s", cs->fd, __func__);
 		goto out;
@@ -839,6 +841,7 @@ out_empty:
 out:
 	free(http_req);
 	dealloc(cs->buf);
+	cksem_post(&cs->sem);
 	return val;
 }
 
diff --git a/src/ckpool.h b/src/ckpool.h
index 573ddc8f..6d0e7348 100644
--- a/src/ckpool.h
+++ b/src/ckpool.h
@@ -79,6 +79,8 @@ struct connsock {
 	char *buf;
 	int bufofs;
 	int buflen;
+	/* Semaphore used to serialise request/responses */
+	sem_t sem;
 };
 
 typedef struct connsock connsock_t;
diff --git a/src/generator.c b/src/generator.c
index 27df2509..2e59816c 100644
--- a/src/generator.c
+++ b/src/generator.c
@@ -1698,6 +1698,8 @@ static int server_mode(ckpool_t *ckp, proc_instance_t *pi)
 		si->auth = ckp->btcdauth[i];
 		si->pass = ckp->btcdpass[i];
 		si->notify = ckp->btcdnotify[i];
+		cksem_init(&si->cs.sem);
+		cksem_post(&si->cs.sem);
 	}
 
 	ret = gen_loop(pi);