diff --git a/modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache b/modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache
index 103c33e0c4e..e632773ee21 100644
--- a/modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache
+++ b/modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache
@@ -336,8 +336,20 @@ cJSON *{{classname}}_convertToJSON({{classname}}_t *{{classname}}) {
         goto fail;
     }
     {{/isEnum}}
+    {{#isEnum}}
+    if ({{projectName}}_{{classVarName}}_{{enumName}}_NULL == {{{classname}}}->{{{name}}}) {
+        goto fail;
+    }
+    {{/isEnum}}
+    {{/required}}
+    {{^required}}
+    {{^isEnum}}
+    if({{{classname}}}->{{{name}}}) {
+    {{/isEnum}}
+    {{#isEnum}}
+    if({{{classname}}}->{{{name}}} != {{projectName}}_{{classVarName}}_{{enumName}}_NULL) {
+    {{/isEnum}}
     {{/required}}
-    {{^required}}{{^isEnum}}if({{{classname}}}->{{{name}}}) { {{/isEnum}}{{/required}}
     {{^isContainer}}
     {{#isPrimitiveType}}
     {{#isNumeric}}
@@ -536,7 +548,7 @@ cJSON *{{classname}}_convertToJSON({{classname}}_t *{{classname}}) {
     {{/isMap}}
     {{/isContainer}}
     {{^required}}
-    {{^isEnum}} } {{/isEnum}}
+    }
     {{/required}}
 
     {{/vars}}
diff --git a/samples/client/petstore/c/model/api_response.c b/samples/client/petstore/c/model/api_response.c
index f783fd84854..117be5cb40d 100644
--- a/samples/client/petstore/c/model/api_response.c
+++ b/samples/client/petstore/c/model/api_response.c
@@ -42,27 +42,27 @@ cJSON *api_response_convertToJSON(api_response_t *api_response) {
     cJSON *item = cJSON_CreateObject();
 
     // api_response->code
-    if(api_response->code) { 
+    if(api_response->code) {
     if(cJSON_AddNumberToObject(item, "code", api_response->code) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // api_response->type
-    if(api_response->type) { 
+    if(api_response->type) {
     if(cJSON_AddStringToObject(item, "type", api_response->type) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // api_response->message
-    if(api_response->message) { 
+    if(api_response->message) {
     if(cJSON_AddStringToObject(item, "message", api_response->message) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
     return item;
 fail:
diff --git a/samples/client/petstore/c/model/category.c b/samples/client/petstore/c/model/category.c
index c192b2b9605..34f5a804bdb 100644
--- a/samples/client/petstore/c/model/category.c
+++ b/samples/client/petstore/c/model/category.c
@@ -36,19 +36,19 @@ cJSON *category_convertToJSON(category_t *category) {
     cJSON *item = cJSON_CreateObject();
 
     // category->id
-    if(category->id) { 
+    if(category->id) {
     if(cJSON_AddNumberToObject(item, "id", category->id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // category->name
-    if(category->name) { 
+    if(category->name) {
     if(cJSON_AddStringToObject(item, "name", category->name) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
     return item;
 fail:
diff --git a/samples/client/petstore/c/model/order.c b/samples/client/petstore/c/model/order.c
index f9e51845273..132aa09405c 100644
--- a/samples/client/petstore/c/model/order.c
+++ b/samples/client/petstore/c/model/order.c
@@ -61,52 +61,52 @@ cJSON *order_convertToJSON(order_t *order) {
     cJSON *item = cJSON_CreateObject();
 
     // order->id
-    if(order->id) { 
+    if(order->id) {
     if(cJSON_AddNumberToObject(item, "id", order->id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // order->pet_id
-    if(order->pet_id) { 
+    if(order->pet_id) {
     if(cJSON_AddNumberToObject(item, "petId", order->pet_id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // order->quantity
-    if(order->quantity) { 
+    if(order->quantity) {
     if(cJSON_AddNumberToObject(item, "quantity", order->quantity) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // order->ship_date
-    if(order->ship_date) { 
+    if(order->ship_date) {
     if(cJSON_AddStringToObject(item, "shipDate", order->ship_date) == NULL) {
     goto fail; //Date-Time
     }
-     } 
+    }
 
 
     // order->status
-    
+    if(order->status != openapi_petstore_order_STATUS_NULL) {
     if(cJSON_AddStringToObject(item, "status", statusorder_ToString(order->status)) == NULL)
     {
     goto fail; //Enum
     }
-    
+    }
 
 
     // order->complete
-    if(order->complete) { 
+    if(order->complete) {
     if(cJSON_AddBoolToObject(item, "complete", order->complete) == NULL) {
     goto fail; //Bool
     }
-     } 
+    }
 
     return item;
 fail:
diff --git a/samples/client/petstore/c/model/pet.c b/samples/client/petstore/c/model/pet.c
index 278854ce519..d46ad06b101 100644
--- a/samples/client/petstore/c/model/pet.c
+++ b/samples/client/petstore/c/model/pet.c
@@ -79,15 +79,15 @@ cJSON *pet_convertToJSON(pet_t *pet) {
     cJSON *item = cJSON_CreateObject();
 
     // pet->id
-    if(pet->id) { 
+    if(pet->id) {
     if(cJSON_AddNumberToObject(item, "id", pet->id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // pet->category
-    if(pet->category) { 
+    if(pet->category) {
     cJSON *category_local_JSON = category_convertToJSON(pet->category);
     if(category_local_JSON == NULL) {
     goto fail; //model
@@ -96,14 +96,13 @@ cJSON *pet_convertToJSON(pet_t *pet) {
     if(item->child == NULL) {
     goto fail;
     }
-     } 
+    }
 
 
     // pet->name
     if (!pet->name) {
         goto fail;
     }
-    
     if(cJSON_AddStringToObject(item, "name", pet->name) == NULL) {
     goto fail; //String
     }
@@ -113,7 +112,6 @@ cJSON *pet_convertToJSON(pet_t *pet) {
     if (!pet->photo_urls) {
         goto fail;
     }
-    
     cJSON *photo_urls = cJSON_AddArrayToObject(item, "photoUrls");
     if(photo_urls == NULL) {
         goto fail; //primitive container
@@ -129,7 +127,7 @@ cJSON *pet_convertToJSON(pet_t *pet) {
 
 
     // pet->tags
-    if(pet->tags) { 
+    if(pet->tags) {
     cJSON *tags = cJSON_AddArrayToObject(item, "tags");
     if(tags == NULL) {
     goto fail; //nonprimitive container
@@ -145,16 +143,16 @@ cJSON *pet_convertToJSON(pet_t *pet) {
     cJSON_AddItemToArray(tags, itemLocal);
     }
     }
-     } 
+    }
 
 
     // pet->status
-    
+    if(pet->status != openapi_petstore_pet_STATUS_NULL) {
     if(cJSON_AddStringToObject(item, "status", statuspet_ToString(pet->status)) == NULL)
     {
     goto fail; //Enum
     }
-    
+    }
 
     return item;
 fail:
diff --git a/samples/client/petstore/c/model/tag.c b/samples/client/petstore/c/model/tag.c
index 00b62ddb076..ab3f4cce1fc 100644
--- a/samples/client/petstore/c/model/tag.c
+++ b/samples/client/petstore/c/model/tag.c
@@ -36,19 +36,19 @@ cJSON *tag_convertToJSON(tag_t *tag) {
     cJSON *item = cJSON_CreateObject();
 
     // tag->id
-    if(tag->id) { 
+    if(tag->id) {
     if(cJSON_AddNumberToObject(item, "id", tag->id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // tag->name
-    if(tag->name) { 
+    if(tag->name) {
     if(cJSON_AddStringToObject(item, "name", tag->name) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
     return item;
 fail:
diff --git a/samples/client/petstore/c/model/user.c b/samples/client/petstore/c/model/user.c
index eab6606225d..5e5b9b39b03 100644
--- a/samples/client/petstore/c/model/user.c
+++ b/samples/client/petstore/c/model/user.c
@@ -68,67 +68,67 @@ cJSON *user_convertToJSON(user_t *user) {
     cJSON *item = cJSON_CreateObject();
 
     // user->id
-    if(user->id) { 
+    if(user->id) {
     if(cJSON_AddNumberToObject(item, "id", user->id) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
 
     // user->username
-    if(user->username) { 
+    if(user->username) {
     if(cJSON_AddStringToObject(item, "username", user->username) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->first_name
-    if(user->first_name) { 
+    if(user->first_name) {
     if(cJSON_AddStringToObject(item, "firstName", user->first_name) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->last_name
-    if(user->last_name) { 
+    if(user->last_name) {
     if(cJSON_AddStringToObject(item, "lastName", user->last_name) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->email
-    if(user->email) { 
+    if(user->email) {
     if(cJSON_AddStringToObject(item, "email", user->email) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->password
-    if(user->password) { 
+    if(user->password) {
     if(cJSON_AddStringToObject(item, "password", user->password) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->phone
-    if(user->phone) { 
+    if(user->phone) {
     if(cJSON_AddStringToObject(item, "phone", user->phone) == NULL) {
     goto fail; //String
     }
-     } 
+    }
 
 
     // user->user_status
-    if(user->user_status) { 
+    if(user->user_status) {
     if(cJSON_AddNumberToObject(item, "userStatus", user->user_status) == NULL) {
     goto fail; //Numeric
     }
-     } 
+    }
 
     return item;
 fail: