diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/vertx/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/vertx/ApiClient.mustache
index b2e7f289bd3e..9f3101629179 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/vertx/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/vertx/ApiClient.mustache
@@ -731,7 +731,19 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
             config.put("userAgent", "{{{httpUserAgent}}}{{^httpUserAgent}}OpenAPI-Generator/{{{artifactVersion}}}/java{{/httpUserAgent}}");
         }
 
-        return WebClient.create(vertx, new WebClientOptions(config){{#useVertx5}}, new PoolOptions(poolConfig){{/useVertx5}});
+        {{#useVertx5}}
+        // PoolOptions(JsonObject) does not initialize defaults first (unlike its
+        // no-arg constructor and unlike WebClientOptions(JsonObject)), so building
+        // it directly from an empty or partial poolConfig leaves fields such as
+        // maxLifetimeUnit null and triggers a NullPointerException when the
+        // WebClient is created. Overlay poolConfig on the serialized defaults so
+        // absent keys keep their documented values.
+        PoolOptions poolOptions = new PoolOptions(new PoolOptions().toJson().mergeIn(poolConfig));
+        return WebClient.create(vertx, new WebClientOptions(config), poolOptions);
+        {{/useVertx5}}
+        {{^useVertx5}}
+        return WebClient.create(vertx, new WebClientOptions(config));
+        {{/useVertx5}}
     }
 
 
diff --git a/samples/client/petstore/java/vertx5-supportVertxFuture/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/vertx5-supportVertxFuture/src/main/java/org/openapitools/client/ApiClient.java
index 673930796e3c..7398295a08bf 100644
--- a/samples/client/petstore/java/vertx5-supportVertxFuture/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/vertx5-supportVertxFuture/src/main/java/org/openapitools/client/ApiClient.java
@@ -664,7 +664,14 @@ protected WebClient buildWebClient(Vertx vertx, JsonObject config, JsonObject po
             config.put("userAgent", "OpenAPI-Generator/1.0.0/java");
         }
 
-        return WebClient.create(vertx, new WebClientOptions(config), new PoolOptions(poolConfig));
+        // PoolOptions(JsonObject) does not initialize defaults first (unlike its
+        // no-arg constructor and unlike WebClientOptions(JsonObject)), so building
+        // it directly from an empty or partial poolConfig leaves fields such as
+        // maxLifetimeUnit null and triggers a NullPointerException when the
+        // WebClient is created. Overlay poolConfig on the serialized defaults so
+        // absent keys keep their documented values.
+        PoolOptions poolOptions = new PoolOptions(new PoolOptions().toJson().mergeIn(poolConfig));
+        return WebClient.create(vertx, new WebClientOptions(config), poolOptions);
     }
 
 
diff --git a/samples/client/petstore/java/vertx5/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/vertx5/src/main/java/org/openapitools/client/ApiClient.java
index 673930796e3c..7398295a08bf 100644
--- a/samples/client/petstore/java/vertx5/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/vertx5/src/main/java/org/openapitools/client/ApiClient.java
@@ -664,7 +664,14 @@ protected WebClient buildWebClient(Vertx vertx, JsonObject config, JsonObject po
             config.put("userAgent", "OpenAPI-Generator/1.0.0/java");
         }
 
-        return WebClient.create(vertx, new WebClientOptions(config), new PoolOptions(poolConfig));
+        // PoolOptions(JsonObject) does not initialize defaults first (unlike its
+        // no-arg constructor and unlike WebClientOptions(JsonObject)), so building
+        // it directly from an empty or partial poolConfig leaves fields such as
+        // maxLifetimeUnit null and triggers a NullPointerException when the
+        // WebClient is created. Overlay poolConfig on the serialized defaults so
+        // absent keys keep their documented values.
+        PoolOptions poolOptions = new PoolOptions(new PoolOptions().toJson().mergeIn(poolConfig));
+        return WebClient.create(vertx, new WebClientOptions(config), poolOptions);
     }