diff --git a/simulate/simulate.cc b/simulate/simulate.cc
index 808a1718..2f54878b 100644
--- a/simulate/simulate.cc
+++ b/simulate/simulate.cc
@@ -84,7 +84,7 @@ const mjuiDef defFile[] = {
   {mjITEM_BUTTON,    "Print model",   2, nullptr,                    "CM"},
   {mjITEM_BUTTON,    "Print data",    2, nullptr,                    "CD"},
   {mjITEM_BUTTON,    "Quit",          1, nullptr,                    "CQ"},
-  {mjITEM_BUTTON,    "Screenshot",    2, NULL,                       "CP"},
+  {mjITEM_BUTTON,    "Screenshot",    2, nullptr,                    "CP"},
   {mjITEM_END}
 };
 
@@ -1088,7 +1088,7 @@ void uiEvent(mjuiState* state) {
         {
           const std::string path = getSavePath("mjmodel.mjb");
           if (!path.empty()) {
-            mj_saveModel(m, path.c_str(), NULL, 0);
+            mj_saveModel(m, path.c_str(), nullptr, 0);
           }
         }
         break;
@@ -1836,7 +1836,7 @@ void Simulate::render() {
     if (!rgb) {
       mju_error("could not allocate buffer for screenshot");
     }
-    mjr_readPixels(rgb.get(), NULL, uistate.rect[0], &con);
+    mjr_readPixels(rgb.get(), nullptr, uistate.rect[0], &con);
 
     // flip up-down
     for (int r = 0; r < h/2; ++r) {
diff --git a/simulate/uitools.cc b/simulate/uitools.cc
index ef7bfa99..362fd544 100644
--- a/simulate/uitools.cc
+++ b/simulate/uitools.cc
@@ -314,17 +314,17 @@ void uiClearCallback(GLFWwindow* wnd) {
   // clear container
   if (Glfw().glfwGetWindowUserPointer(wnd)) {
     mju_free(Glfw().glfwGetWindowUserPointer(wnd));
-    Glfw().glfwSetWindowUserPointer(wnd, NULL);
+    Glfw().glfwSetWindowUserPointer(wnd, nullptr);
   }
 
   // clear internal callbacks
-  Glfw().glfwSetKeyCallback(wnd, NULL);
-  Glfw().glfwSetCursorPosCallback(wnd, NULL);
-  Glfw().glfwSetMouseButtonCallback(wnd, NULL);
-  Glfw().glfwSetScrollCallback(wnd, NULL);
-  Glfw().glfwSetWindowSizeCallback(wnd, NULL);
-  Glfw().glfwSetWindowRefreshCallback(wnd, NULL);
-  Glfw().glfwSetDropCallback(wnd, NULL);
+  Glfw().glfwSetKeyCallback(wnd, nullptr);
+  Glfw().glfwSetCursorPosCallback(wnd, nullptr);
+  Glfw().glfwSetMouseButtonCallback(wnd, nullptr);
+  Glfw().glfwSetScrollCallback(wnd, nullptr);
+  Glfw().glfwSetWindowSizeCallback(wnd, nullptr);
+  Glfw().glfwSetWindowRefreshCallback(wnd, nullptr);
+  Glfw().glfwSetDropCallback(wnd, nullptr);
 }