diff --git a/scripts/generateTypes.ts b/scripts/generateTypes.ts
index 0d23d2a..06ac216 100644
--- a/scripts/generateTypes.ts
+++ b/scripts/generateTypes.ts
@@ -273,7 +273,9 @@ async function main() {
         namedFields[0].name.endsWith('val') &&
         namedFields[0].name.length > 3
       ) {
-        constTypes.push(typeName)
+        // A_Const double-wraps the value for some strange reason.
+        //   https://github.com/pganalyze/libpg_query/issues/265
+        constTypes.push(`{ ${namedFields[0].name}: ${typeName} }`)
       }
 
       const fieldMetadata = nodeFieldsByTag[typeName]
@@ -428,6 +430,9 @@ async function main() {
     }
   })
 
+  // A_Const can represent a NULL value.
+  constTypes.push('{ isnull: true }')
+
   code += '\n'
   code += `/** A qualified name for referencing a database object, e.g. "public.my_table" */\n`
   code += 'export type QualifiedName = { String: String }[]\n'
diff --git a/src/lib/ast.ts b/src/lib/ast.ts
index 6a0480c..5909708 100644
--- a/src/lib/ast.ts
+++ b/src/lib/ast.ts
@@ -6779,7 +6779,13 @@ export type QualifiedName = { String: String }[]
 
 export type List<T = Node> = { items: T[] }
 
-export type A_Const = BitString | Boolean | Float | Integer | String
+export type A_Const =
+  | { boolval: Boolean }
+  | { bsval: BitString }
+  | { fval: Float }
+  | { isnull: true }
+  | { ival: Integer }
+  | { sval: String }
 
 /**
  * Expr - generic superclass for executable-expression nodes