diff --git a/src/network.c b/src/network.c
index 1cae5ef..5e646cd 100644
--- a/src/network.c
+++ b/src/network.c
@@ -328,7 +328,9 @@ void transfer_blocking(CURL *curl)
     transfer.transferring = 1;
     curl_easy_setopt(curl, CURLOPT_PRIVATE, &transfer);
 
+    pthread_mutex_lock(&transfer_lock);
     CURLMcode res = curl_multi_add_handle(curl_multi, curl);
+    pthread_mutex_unlock(&transfer_lock);
 
     if(res > 0) {
         fprintf(stderr, "transfer_blocking(): %d, %s\n",
@@ -343,7 +345,10 @@ void transfer_blocking(CURL *curl)
 
 void transfer_nonblocking(CURL *curl)
 {
+    pthread_mutex_lock(&transfer_lock);
     CURLMcode res = curl_multi_add_handle(curl_multi, curl);
+    pthread_mutex_unlock(&transfer_lock);
+
     if(res > 0) {
         fprintf(stderr, "transfer_nonblocking(): %s\n",
                 curl_multi_strerror(res));