diff --git a/core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java b/core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java
index 8e391bd230..695a4411d4 100644
--- a/core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java
+++ b/core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java
@@ -674,7 +674,10 @@ public void writeWithFilter(JSONWriter jsonWriter, Object object, Object fieldNa
                 }
 
                 if (nameFilter != null) {
-                    key = nameFilter.process(object, key, value);
+                    String filteredKey = nameFilter.process(object, key, value);
+                    if (filteredKey != null) {
+                        key = filteredKey;
+                    }
                 }
 
                 if (propertyFilter != null) {
diff --git a/core/src/test/java/com/alibaba/fastjson2/issues_7000/Issue7644.java b/core/src/test/java/com/alibaba/fastjson2/issues_7000/Issue7644.java
new file mode 100644
index 0000000000..587cf5714e
--- /dev/null
+++ b/core/src/test/java/com/alibaba/fastjson2/issues_7000/Issue7644.java
@@ -0,0 +1,32 @@
+package com.alibaba.fastjson2.issues_7000;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.filter.NameFilter;
+import org.junit.jupiter.api.Test;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue7644 {
+    @Test
+    public void testNameFilterNullReturnPreservesMapKey() {
+        Map<String, String> map = new LinkedHashMap<>();
+        map.put("aaaa", "value");
+
+        NameFilter nameFilter = (object, name, value) -> null;
+
+        assertEquals("{\"aaaa\":\"value\"}", JSON.toJSONString(map, nameFilter));
+    }
+
+    @Test
+    public void testNameFilterReturnValueRenamesMapKey() {
+        Map<String, String> map = new LinkedHashMap<>();
+        map.put("aaaa", "value");
+
+        NameFilter nameFilter = (object, name, value) -> "renamed_" + name;
+
+        assertEquals("{\"renamed_aaaa\":\"value\"}", JSON.toJSONString(map, nameFilter));
+    }
+}