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 65685b8c6787..11dcb74c884c 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
@@ -635,7 +635,7 @@ fail:
     {{^isEnum}}
     {{#isString}}
     {{^required}}if ({{{name}}}) { {{/required}}
-    if(!cJSON_IsString({{{name}}}))
+    if(!cJSON_IsString({{{name}}}){{^required}} && !cJSON_IsNull({{{name}}}){{/required}})
     {
     goto end; //String
     }
@@ -880,7 +880,7 @@ fail:
         {{/isEnum}}
         {{^isEnum}}
         {{#isString}}
-        {{^required}}{{{name}}} ? {{/required}}strdup({{{name}}}->valuestring){{^required}} : NULL{{/required}}{{^-last}},{{/-last}}
+        {{^required}}{{{name}}} && !cJSON_IsNull({{{name}}}) ? {{/required}}strdup({{{name}}}->valuestring){{^required}} : NULL{{/required}}{{^-last}},{{/-last}}
         {{/isString}}
         {{/isEnum}}
         {{#isByteArray}}
diff --git a/samples/client/petstore/c/model/api_response.c b/samples/client/petstore/c/model/api_response.c
index 117be5cb40d5..a1636e35b49b 100644
--- a/samples/client/petstore/c/model/api_response.c
+++ b/samples/client/petstore/c/model/api_response.c
@@ -88,7 +88,7 @@ api_response_t *api_response_parseFromJSON(cJSON *api_responseJSON){
     // api_response->type
     cJSON *type = cJSON_GetObjectItemCaseSensitive(api_responseJSON, "type");
     if (type) { 
-    if(!cJSON_IsString(type))
+    if(!cJSON_IsString(type) && !cJSON_IsNull(type))
     {
     goto end; //String
     }
@@ -97,7 +97,7 @@ api_response_t *api_response_parseFromJSON(cJSON *api_responseJSON){
     // api_response->message
     cJSON *message = cJSON_GetObjectItemCaseSensitive(api_responseJSON, "message");
     if (message) { 
-    if(!cJSON_IsString(message))
+    if(!cJSON_IsString(message) && !cJSON_IsNull(message))
     {
     goto end; //String
     }
@@ -106,8 +106,8 @@ api_response_t *api_response_parseFromJSON(cJSON *api_responseJSON){
 
     api_response_local_var = api_response_create (
         code ? code->valuedouble : 0,
-        type ? strdup(type->valuestring) : NULL,
-        message ? strdup(message->valuestring) : NULL
+        type && !cJSON_IsNull(type) ? strdup(type->valuestring) : NULL,
+        message && !cJSON_IsNull(message) ? strdup(message->valuestring) : NULL
         );
 
     return api_response_local_var;
diff --git a/samples/client/petstore/c/model/category.c b/samples/client/petstore/c/model/category.c
index 34f5a804bdb2..3f6c32da5278 100644
--- a/samples/client/petstore/c/model/category.c
+++ b/samples/client/petstore/c/model/category.c
@@ -74,7 +74,7 @@ category_t *category_parseFromJSON(cJSON *categoryJSON){
     // category->name
     cJSON *name = cJSON_GetObjectItemCaseSensitive(categoryJSON, "name");
     if (name) { 
-    if(!cJSON_IsString(name))
+    if(!cJSON_IsString(name) && !cJSON_IsNull(name))
     {
     goto end; //String
     }
@@ -83,7 +83,7 @@ category_t *category_parseFromJSON(cJSON *categoryJSON){
 
     category_local_var = category_create (
         id ? id->valuedouble : 0,
-        name ? strdup(name->valuestring) : NULL
+        name && !cJSON_IsNull(name) ? strdup(name->valuestring) : NULL
         );
 
     return category_local_var;
diff --git a/samples/client/petstore/c/model/tag.c b/samples/client/petstore/c/model/tag.c
index ab3f4cce1fc6..00d73b2d2473 100644
--- a/samples/client/petstore/c/model/tag.c
+++ b/samples/client/petstore/c/model/tag.c
@@ -74,7 +74,7 @@ tag_t *tag_parseFromJSON(cJSON *tagJSON){
     // tag->name
     cJSON *name = cJSON_GetObjectItemCaseSensitive(tagJSON, "name");
     if (name) { 
-    if(!cJSON_IsString(name))
+    if(!cJSON_IsString(name) && !cJSON_IsNull(name))
     {
     goto end; //String
     }
@@ -83,7 +83,7 @@ tag_t *tag_parseFromJSON(cJSON *tagJSON){
 
     tag_local_var = tag_create (
         id ? id->valuedouble : 0,
-        name ? strdup(name->valuestring) : NULL
+        name && !cJSON_IsNull(name) ? strdup(name->valuestring) : NULL
         );
 
     return tag_local_var;
diff --git a/samples/client/petstore/c/model/user.c b/samples/client/petstore/c/model/user.c
index 5e5b9b39b03f..eb188153ccf0 100644
--- a/samples/client/petstore/c/model/user.c
+++ b/samples/client/petstore/c/model/user.c
@@ -154,7 +154,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->username
     cJSON *username = cJSON_GetObjectItemCaseSensitive(userJSON, "username");
     if (username) { 
-    if(!cJSON_IsString(username))
+    if(!cJSON_IsString(username) && !cJSON_IsNull(username))
     {
     goto end; //String
     }
@@ -163,7 +163,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->first_name
     cJSON *first_name = cJSON_GetObjectItemCaseSensitive(userJSON, "firstName");
     if (first_name) { 
-    if(!cJSON_IsString(first_name))
+    if(!cJSON_IsString(first_name) && !cJSON_IsNull(first_name))
     {
     goto end; //String
     }
@@ -172,7 +172,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->last_name
     cJSON *last_name = cJSON_GetObjectItemCaseSensitive(userJSON, "lastName");
     if (last_name) { 
-    if(!cJSON_IsString(last_name))
+    if(!cJSON_IsString(last_name) && !cJSON_IsNull(last_name))
     {
     goto end; //String
     }
@@ -181,7 +181,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->email
     cJSON *email = cJSON_GetObjectItemCaseSensitive(userJSON, "email");
     if (email) { 
-    if(!cJSON_IsString(email))
+    if(!cJSON_IsString(email) && !cJSON_IsNull(email))
     {
     goto end; //String
     }
@@ -190,7 +190,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->password
     cJSON *password = cJSON_GetObjectItemCaseSensitive(userJSON, "password");
     if (password) { 
-    if(!cJSON_IsString(password))
+    if(!cJSON_IsString(password) && !cJSON_IsNull(password))
     {
     goto end; //String
     }
@@ -199,7 +199,7 @@ user_t *user_parseFromJSON(cJSON *userJSON){
     // user->phone
     cJSON *phone = cJSON_GetObjectItemCaseSensitive(userJSON, "phone");
     if (phone) { 
-    if(!cJSON_IsString(phone))
+    if(!cJSON_IsString(phone) && !cJSON_IsNull(phone))
     {
     goto end; //String
     }
@@ -217,12 +217,12 @@ user_t *user_parseFromJSON(cJSON *userJSON){
 
     user_local_var = user_create (
         id ? id->valuedouble : 0,
-        username ? strdup(username->valuestring) : NULL,
-        first_name ? strdup(first_name->valuestring) : NULL,
-        last_name ? strdup(last_name->valuestring) : NULL,
-        email ? strdup(email->valuestring) : NULL,
-        password ? strdup(password->valuestring) : NULL,
-        phone ? strdup(phone->valuestring) : NULL,
+        username && !cJSON_IsNull(username) ? strdup(username->valuestring) : NULL,
+        first_name && !cJSON_IsNull(first_name) ? strdup(first_name->valuestring) : NULL,
+        last_name && !cJSON_IsNull(last_name) ? strdup(last_name->valuestring) : NULL,
+        email && !cJSON_IsNull(email) ? strdup(email->valuestring) : NULL,
+        password && !cJSON_IsNull(password) ? strdup(password->valuestring) : NULL,
+        phone && !cJSON_IsNull(phone) ? strdup(phone->valuestring) : NULL,
         user_status ? user_status->valuedouble : 0
         );