error can be anything, you decide

I was wrong. 'Error' is whatever YOUR validate and asyncValidate functions will return.

redux-form/redux-form.d.ts

@@ -39,7 +39,7 @@ declare module 'redux-form' {
      * The error for this field if its value is not passing validation. Both
      * synchronous and asynchronous validation errors will be reported here.
      */
-    error?: string[];
+    error?: any;
 
     /**
      * The value for this field as supplied in initialValues to the form.
@@ -163,7 +163,7 @@ declare module 'redux-form' {
      * result from the synchronous validation function, the asynchronous
      * validation, or the rejected promise from onSubmit.
      */
-    error?: string[];
+    error?: any;
 
     /**
      * The form data, in the form { field1: <Object>, field2: <Object> }. The