diff --git a/complex-types.d.ts b/complex-types.d.ts
index 8c8bfd0..732d5b2 100644
--- a/complex-types.d.ts
+++ b/complex-types.d.ts
@@ -1,5 +1,13 @@
 import type {Literal} from 'mdast'
 
+declare module 'mdast-util-from-markdown' {
+  // eslint-disable-next-line @typescript-eslint/consistent-type-definitions
+  interface CompileData {
+    // Register a new field.
+    mathFlowInside?: boolean | undefined
+  }
+}
+
 // eslint-disable-next-line @typescript-eslint/consistent-type-definitions
 export interface Math extends Literal {
   type: 'math'