diff --git a/src/core/ngx_cycle.c b/src/core/ngx_cycle.c
index e9dfdfc2a..eec7da592 100644
--- a/src/core/ngx_cycle.c
+++ b/src/core/ngx_cycle.c
@@ -1207,6 +1207,10 @@ ngx_shared_memory_add(ngx_conf_t *cf, ngx_str_t *name, size_t size, void *tag)
             return NULL;
         }
 
+        if (shm_zone[i].shm.size == 0) {
+            shm_zone[i].shm.size = size;
+        }
+
         if (size && size != shm_zone[i].shm.size) {
             ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
                             "the size %uz of shared memory zone \"%V\" "