diff --git a/libqpdf/QPDFAcroFormDocumentHelper.cc b/libqpdf/QPDFAcroFormDocumentHelper.cc
index 65f103c..ac65a73 100644
--- a/libqpdf/QPDFAcroFormDocumentHelper.cc
+++ b/libqpdf/QPDFAcroFormDocumentHelper.cc
@@ -1,5 +1,7 @@
 #include <qpdf/QPDFAcroFormDocumentHelper.hh>
 
+#include <qpdf/FormField.hh>
+
 #include <qpdf/Pl_Buffer.hh>
 #include <qpdf/QPDFObjectHandle_private.hh>
 #include <qpdf/QPDFPageDocumentHelper.hh>
@@ -14,29 +16,20 @@
 using namespace qpdf;
 using namespace std::literals;
 
-class QPDFAcroFormDocumentHelper::Members
+using AcroForm = impl::AcroForm;
+
+class QPDFAcroFormDocumentHelper::Members: public AcroForm
 {
   public:
-    Members() = default;
-    Members(Members const&) = delete;
-    ~Members() = default;
-
-    struct FieldData
+    Members(QPDF& qpdf) :
+        AcroForm(qpdf.doc())
     {
-        std::vector<QPDFAnnotationObjectHelper> annotations;
-        std::string name;
-    };
-
-    bool cache_valid{false};
-    std::map<QPDFObjGen, FieldData> field_to;
-    std::map<QPDFObjGen, QPDFFormFieldObjectHelper> annotation_to_field;
-    std::map<std::string, std::set<QPDFObjGen>> name_to_fields;
-    std::set<QPDFObjGen> bad_fields;
+    }
 };
 
 QPDFAcroFormDocumentHelper::QPDFAcroFormDocumentHelper(QPDF& qpdf) :
     QPDFDocumentHelper(qpdf),
-    m(std::make_shared<Members>())
+    m(std::make_shared<Members>(qpdf))
 {
     // We have to analyze up front. Otherwise, when we are adding annotations and fields, we are in
     // a temporarily unstable configuration where some widget annotations are not reachable.
@@ -62,6 +55,7 @@ QPDFAcroFormDocumentHelper::invalidateCache()
     m->cache_valid = false;
     m->field_to.clear();
     m->annotation_to_field.clear();
+    m->bad_fields.clear();
 }
 
 bool
diff --git a/libqpdf/qpdf/FormField.hh b/libqpdf/qpdf/FormField.hh
index 6a15023..8ea8a10 100644
--- a/libqpdf/qpdf/FormField.hh
+++ b/libqpdf/qpdf/FormField.hh
@@ -3,6 +3,7 @@
 
 #include <qpdf/QPDFObjectHandle_private.hh>
 #include <qpdf/QPDFObjectHelper.hh>
+#include <qpdf/QPDF_private.hh>
 
 #include <vector>
 
@@ -10,6 +11,34 @@ class QPDFAnnotationObjectHelper;
 
 namespace qpdf::impl
 {
+    class AcroForm: public Doc::Common
+    {
+      public:
+        AcroForm() = delete;
+        AcroForm(AcroForm const&) = delete;
+        AcroForm(AcroForm&&) = delete;
+        AcroForm& operator=(AcroForm const&) = delete;
+        AcroForm& operator=(AcroForm&&) = delete;
+        ~AcroForm() = default;
+
+        AcroForm(impl::Doc& doc) :
+            Common(doc)
+        {
+        }
+
+        struct FieldData
+        {
+            std::vector<QPDFAnnotationObjectHelper> annotations;
+            std::string name;
+        };
+
+        bool cache_valid{false};
+        std::map<QPDFObjGen, FieldData> field_to;
+        std::map<QPDFObjGen, QPDFFormFieldObjectHelper> annotation_to_field;
+        std::map<std::string, std::set<QPDFObjGen>> name_to_fields;
+        std::set<QPDFObjGen> bad_fields;
+    }; // class AcroForm
+
     /// @class FormNode
     /// @brief Represents a node in the interactive forms tree of a PDF document.
     ///
@@ -390,7 +419,7 @@ namespace qpdf::impl
         getFontFromResource(QPDFObjectHandle resources, std::string const& font_name);
 
         static const QPDFObjectHandle null_oh;
-    };
+    }; // class FormNode
 } // namespace qpdf::impl
 
 #endif // FORMFIELD_HH