diff --git a/springdoc-openapi-common/src/main/java/org/springdoc/core/OpenAPIService.java b/springdoc-openapi-common/src/main/java/org/springdoc/core/OpenAPIService.java
index 6a60334f8..36322a707 100644
--- a/springdoc-openapi-common/src/main/java/org/springdoc/core/OpenAPIService.java
+++ b/springdoc-openapi-common/src/main/java/org/springdoc/core/OpenAPIService.java
@@ -37,6 +37,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import com.fasterxml.jackson.annotation.JsonInclude.Include;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import io.swagger.v3.core.jackson.TypeNameResolver;
@@ -249,6 +250,9 @@ public OpenAPI build(Locale locale) {
 			try {
 				ObjectMapper objectMapper = ObjectMapperProvider.createJson(springDocConfigProperties);
 				calculatedOpenAPI = objectMapper.readValue(objectMapper.writeValueAsString(openAPI), OpenAPI.class);
+				objectMapper.setSerializationInclusion(Include.ALWAYS);
+				Map extensionsClone = objectMapper.readValue(objectMapper.writeValueAsString(openAPI.getExtensions()), Map.class);
+				calculatedOpenAPI.extensions(extensionsClone);
 			}
 			catch (JsonProcessingException e) {
 				LOGGER.warn("Json Processing Exception occurred: {}", e.getMessage());
diff --git a/springdoc-openapi-webmvc-core/src/test/java/test/org/springdoc/api/v30/app173/SpringDocApp173Test.java b/springdoc-openapi-webmvc-core/src/test/java/test/org/springdoc/api/v30/app173/SpringDocApp173Test.java
index 39df08dbc..1990a052b 100644
--- a/springdoc-openapi-webmvc-core/src/test/java/test/org/springdoc/api/v30/app173/SpringDocApp173Test.java
+++ b/springdoc-openapi-webmvc-core/src/test/java/test/org/springdoc/api/v30/app173/SpringDocApp173Test.java
@@ -22,6 +22,7 @@
 
 package test.org.springdoc.api.v30.app173;
 
+import java.util.Collections;
 import java.util.Locale;
 
 import io.swagger.v3.oas.models.OpenAPI;
@@ -65,7 +66,7 @@ private void testApp(Locale locale) throws Exception {
 	static class SpringDocTestApp {
 		@Bean
 		public OpenAPI openAPI() {
-			return new OpenAPI();
+			return new OpenAPI().extensions(Collections.singletonMap("TEST", "HELLO"));
 		}
 	}
 
diff --git a/springdoc-openapi-webmvc-core/src/test/resources/results/3.0.1/app173.json b/springdoc-openapi-webmvc-core/src/test/resources/results/3.0.1/app173.json
index a8128527a..616425fa4 100644
--- a/springdoc-openapi-webmvc-core/src/test/resources/results/3.0.1/app173.json
+++ b/springdoc-openapi-webmvc-core/src/test/resources/results/3.0.1/app173.json
@@ -48,5 +48,6 @@
       }
     }
   },
-  "components": {}
+  "components": {},
+  "TEST": "HELLO"
 }