diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 0f6744d..11dbb14 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1986,7 +1986,8 @@
 for `a` of struct type:
 
 - the index `x` unified with `string` must be concrete.
-- the value of the field named `x` of struct `a`, if this field exists
+- the value of the regular and non-optional field named `x` of struct `a`,
+  if this field exists
 - bottom (an error), otherwise