diff --git a/grammar/abnf-grammar.txt b/grammar/abnf-grammar.txt
index 3786d3906a..fdca71e60c 100644
--- a/grammar/abnf-grammar.txt
+++ b/grammar/abnf-grammar.txt
@@ -678,8 +678,10 @@ tuple-type = "(" [ type 1*( "," type ) ] ")"
 
 array-type = "[" type ";" array-dimensions "]"
 
-array-dimensions = natural
-                 / "(" natural *( "," natural ) ")"
+array-dimension = natural / "_"
+
+array-dimensions = array-dimension
+                 / "(" array-dimension *( "," array-dimension ) ")"
 
 ; Scalar and the remaining types form all the types.