diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 69e3ebc90c..f7fe8931bf 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -602,6 +602,8 @@ install(
     PATTERN "*.h"
     PATTERN "*.pac"
     PATTERN "3rdparty/*" EXCLUDE
+    # Headers used only during build
+    PATTERN "threading/formatters/detail" EXCLUDE
     # The "zeek -> ." symlink isn't needed in the install-tree
     REGEX "${escaped_include_path}$" EXCLUDE
     # FILES_MATCHING creates empty directories:
diff --git a/src/Val.cc b/src/Val.cc
index 102021c8a6..3cb1d08a63 100644
--- a/src/Val.cc
+++ b/src/Val.cc
@@ -39,7 +39,7 @@
 #include "zeek/broker/Data.h"
 #include "zeek/broker/Manager.h"
 #include "zeek/broker/Store.h"
-#include "zeek/threading/formatters/JSON.h"
+#include "zeek/threading/formatters/detail/json.h"
 
 using namespace std;
 
@@ -404,8 +404,8 @@ TableValPtr Val::GetRecordFields()
 
 // This is a static method in this file to avoid including rapidjson's headers in Val.h because
 // they're huge.
-static void BuildJSON(threading::formatter::JSON::NullDoubleWriter& writer, Val* val,
-                      bool only_loggable = false, RE_Matcher* re = nullptr, const string& key = "")
+static void BuildJSON(json::detail::NullDoubleWriter& writer, Val* val, bool only_loggable = false,
+                      RE_Matcher* re = nullptr, const string& key = "")
 	{
 	if ( ! key.empty() )
 		writer.Key(key);
@@ -509,7 +509,7 @@ static void BuildJSON(threading::formatter::JSON::NullDoubleWriter& writer, Val*
 				else
 					{
 					rapidjson::StringBuffer buffer;
-					threading::formatter::JSON::NullDoubleWriter key_writer(buffer);
+					json::detail::NullDoubleWriter key_writer(buffer);
 					BuildJSON(key_writer, entry_key, only_loggable, re);
 					string key_str = buffer.GetString();
 
@@ -612,7 +612,7 @@ static void BuildJSON(threading::formatter::JSON::NullDoubleWriter& writer, Val*
 StringValPtr Val::ToJSON(bool only_loggable, RE_Matcher* re)
 	{
 	rapidjson::StringBuffer buffer;
-	threading::formatter::JSON::NullDoubleWriter writer(buffer);
+	json::detail::NullDoubleWriter writer(buffer);
 
 	BuildJSON(writer, this, only_loggable, re, "");
 
diff --git a/src/threading/formatters/JSON.cc b/src/threading/formatters/JSON.cc
index 9c5825bdfd..25ba007596 100644
--- a/src/threading/formatters/JSON.cc
+++ b/src/threading/formatters/JSON.cc
@@ -16,10 +16,12 @@
 
 #include "zeek/Desc.h"
 #include "zeek/threading/MsgThread.h"
+#include "zeek/threading/formatters/detail/json.h"
 
 namespace zeek::threading::formatter
 	{
 
+// For deprecated NullDoubleWriter
 bool JSON::NullDoubleWriter::Double(double d)
 	{
 	if ( rapidjson::internal::Double(d).IsNanOrInf() )
@@ -39,7 +41,7 @@ JSON::~JSON() { }
 bool JSON::Describe(ODesc* desc, int num_fields, const Field* const* fields, Value** vals) const
 	{
 	rapidjson::StringBuffer buffer;
-	NullDoubleWriter writer(buffer);
+	zeek::json::detail::NullDoubleWriter writer(buffer);
 
 	writer.StartObject();
 
@@ -68,7 +70,7 @@ bool JSON::Describe(ODesc* desc, Value* val, const std::string& name) const
 
 	rapidjson::Document doc;
 	rapidjson::StringBuffer buffer;
-	NullDoubleWriter writer(buffer);
+	zeek::json::detail::NullDoubleWriter writer(buffer);
 
 	writer.StartObject();
 	BuildJSON(writer, val, name);
@@ -85,7 +87,8 @@ Value* JSON::ParseValue(const std::string& s, const std::string& name, TypeTag t
 	return nullptr;
 	}
 
-void JSON::BuildJSON(NullDoubleWriter& writer, Value* val, const std::string& name) const
+void JSON::BuildJSON(zeek::json::detail::NullDoubleWriter& writer, Value* val,
+                     const std::string& name) const
 	{
 	if ( ! name.empty() )
 		writer.Key(name);
diff --git a/src/threading/formatters/JSON.h b/src/threading/formatters/JSON.h
index c946ab7bf2..e76d3e3edf 100644
--- a/src/threading/formatters/JSON.h
+++ b/src/threading/formatters/JSON.h
@@ -3,11 +3,18 @@
 #pragma once
 
 #define RAPIDJSON_HAS_STDSTRING 1
+// Remove in v7.1 when removing NullDoubleWriter below and also remove
+// rapidjson include tweaks from  CMake's dynamic_plugin_base target.
 #include <rapidjson/document.h>
 #include <rapidjson/writer.h>
 
 #include "zeek/threading/Formatter.h"
 
+namespace zeek::json::detail
+	{
+class NullDoubleWriter;
+	}
+
 namespace zeek::threading::formatter
 	{
 
@@ -38,7 +45,8 @@ public:
 	class NullDoubleWriter : public rapidjson::Writer<rapidjson::StringBuffer>
 		{
 	public:
-		NullDoubleWriter(rapidjson::StringBuffer& stream)
+		[[deprecated("Remove in v7.1 - This is an implementation detail.")]] NullDoubleWriter(
+			rapidjson::StringBuffer& stream)
 			: rapidjson::Writer<rapidjson::StringBuffer>(stream)
 			{
 			}
@@ -46,7 +54,8 @@ public:
 		};
 
 private:
-	void BuildJSON(NullDoubleWriter& writer, Value* val, const std::string& name = "") const;
+	void BuildJSON(zeek::json::detail::NullDoubleWriter& writer, Value* val,
+	               const std::string& name = "") const;
 
 	TimeFormat timestamps;
 	bool surrounding_braces;
diff --git a/src/threading/formatters/detail/json.h b/src/threading/formatters/detail/json.h
new file mode 100644
index 0000000000..7ae9f05c70
--- /dev/null
+++ b/src/threading/formatters/detail/json.h
@@ -0,0 +1,27 @@
+// Not installed - used by Val.cc and formatters/JSON.cc only.
+#pragma once
+
+#include <rapidjson/document.h>
+#include <rapidjson/internal/ieee754.h>
+#include <rapidjson/writer.h>
+
+namespace zeek::json::detail
+	{
+// A rapidjson Writer that writes null for inf or nan numbers.
+class NullDoubleWriter : public rapidjson::Writer<rapidjson::StringBuffer>
+	{
+public:
+	explicit NullDoubleWriter(rapidjson::StringBuffer& stream)
+		: rapidjson::Writer<rapidjson::StringBuffer>(stream)
+		{
+		}
+	bool Double(double d)
+		{
+		if ( rapidjson::internal::Double(d).IsNanOrInf() )
+			return rapidjson::Writer<rapidjson::StringBuffer>::Null();
+
+		return rapidjson::Writer<rapidjson::StringBuffer>::Double(d);
+		}
+	};
+
+	}