diff --git a/src/njs_vmcode.c b/src/njs_vmcode.c
index ef0beb5e..1dc35b77 100644
--- a/src/njs_vmcode.c
+++ b/src/njs_vmcode.c
@@ -1055,6 +1055,11 @@ njs_vmcode_array(njs_vm_t *vm, u_char *pc)
 
         if (code->ctor) {
             /* Array of the form [,,,], [1,,]. */
+            if (njs_slow_path(!array->object.fast_array)) {
+                njs_stderror("ARRAYLIT-INVARIANT ctor array literal fill on "
+                             "slow array (start=%p)\n", array->start);
+                abort();
+            }
             value = array->start;
             length = array->length;