diff --git a/glfw/cocoa_monitor.m b/glfw/cocoa_monitor.m
index 70d64e116..fbd01d3d8 100644
--- a/glfw/cocoa_monitor.m
+++ b/glfw/cocoa_monitor.m
@@ -32,6 +32,7 @@
 #include <stdlib.h>
 #include <limits.h>
 #include <math.h>
+#include <assert.h>
 
 #include <IOKit/graphics/IOGraphicsLib.h>
 #include <CoreVideo/CVBase.h>
@@ -635,6 +636,8 @@ void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor, const GLFWgammaramp* ramp)
 GLFWAPI CGDirectDisplayID glfwGetCocoaMonitor(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(kCGNullDirectDisplay);
     return monitor->ns.displayID;
 }
diff --git a/glfw/cocoa_window.m b/glfw/cocoa_window.m
index 0cbc95acc..74fcc4e36 100644
--- a/glfw/cocoa_window.m
+++ b/glfw/cocoa_window.m
@@ -35,6 +35,7 @@
 #import <UniformTypeIdentifiers/UniformTypeIdentifiers.h>
 #include <float.h>
 #include <string.h>
+#include <assert.h>
 
 #define debug debug_rendering
 
@@ -3127,6 +3128,8 @@ VkResult _glfwPlatformCreateWindowSurface(VkInstance instance,
 GLFWAPI id glfwGetCocoaWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(nil);
     return window->ns.object;
 }
diff --git a/glfw/egl_context.c b/glfw/egl_context.c
index b4da38a8c..63198262b 100644
--- a/glfw/egl_context.c
+++ b/glfw/egl_context.c
@@ -790,6 +790,8 @@ GLFWAPI EGLDisplay glfwGetEGLDisplay(void)
 GLFWAPI EGLContext glfwGetEGLContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(EGL_NO_CONTEXT);
 
     if (window->context.client == GLFW_NO_API)
@@ -804,6 +806,8 @@ GLFWAPI EGLContext glfwGetEGLContext(GLFWwindow* handle)
 GLFWAPI EGLSurface glfwGetEGLSurface(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(EGL_NO_SURFACE);
 
     if (window->context.client == GLFW_NO_API)
diff --git a/glfw/glx_context.c b/glfw/glx_context.c
index 849ff910a..48faafc8f 100644
--- a/glfw/glx_context.c
+++ b/glfw/glx_context.c
@@ -658,6 +658,8 @@ bool _glfwChooseVisualGLX(const _GLFWwndconfig* wndconfig UNUSED,
 GLFWAPI GLXContext glfwGetGLXContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (window->context.client == GLFW_NO_API)
@@ -672,6 +674,8 @@ GLFWAPI GLXContext glfwGetGLXContext(GLFWwindow* handle)
 GLFWAPI GLXWindow glfwGetGLXWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(None);
 
     if (window->context.client == GLFW_NO_API)
diff --git a/glfw/nsgl_context.m b/glfw/nsgl_context.m
index 69b656b21..c4c9a739c 100644
--- a/glfw/nsgl_context.m
+++ b/glfw/nsgl_context.m
@@ -27,6 +27,7 @@
 //========================================================================
 
 #include "internal.h"
+#include <assert.h>
 
 
 static void makeContextCurrentNSGL(_GLFWwindow* window)
@@ -319,6 +320,8 @@ bool _glfwCreateContextNSGL(_GLFWwindow* window,
 GLFWAPI id glfwGetNSGLContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(nil);
 
     if (window->context.client == GLFW_NO_API)
diff --git a/glfw/osmesa_context.c b/glfw/osmesa_context.c
index c689b3b31..e688cbe26 100644
--- a/glfw/osmesa_context.c
+++ b/glfw/osmesa_context.c
@@ -352,6 +352,8 @@ GLFWAPI int glfwGetOSMesaDepthBuffer(GLFWwindow* handle,
 GLFWAPI OSMesaContext glfwGetOSMesaContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (window->context.client == GLFW_NO_API)
diff --git a/glfw/wl_monitor.c b/glfw/wl_monitor.c
index f58cb3efb..f1be0f4b4 100644
--- a/glfw/wl_monitor.c
+++ b/glfw/wl_monitor.c
@@ -28,10 +28,10 @@
 
 #include "internal.h"
 
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>
+#include <assert.h>
 
 
 static void outputHandleGeometry(void* data,
@@ -241,6 +241,8 @@ void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor UNUSED,
 GLFWAPI struct wl_output* glfwGetWaylandMonitor(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return monitor->wl.output;
 }
diff --git a/glfw/wl_window.c b/glfw/wl_window.c
index 247397d66..60fb60881 100644
--- a/glfw/wl_window.c
+++ b/glfw/wl_window.c
@@ -40,6 +40,7 @@
 #include <string.h>
 #include <fcntl.h>
 #include <sys/mman.h>
+#include <assert.h>
 
 #define debug debug_rendering
 
@@ -2869,6 +2870,8 @@ GLFWAPI struct wl_display* glfwGetWaylandDisplay(void)
 GLFWAPI struct wl_surface* glfwGetWaylandWindow(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
     return window->wl.surface;
 }
diff --git a/glfw/x11_monitor.c b/glfw/x11_monitor.c
index 4c1a67d1e..aa7cc0ddb 100644
--- a/glfw/x11_monitor.c
+++ b/glfw/x11_monitor.c
@@ -33,6 +33,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>
+#include <assert.h>
 
 
 // Check whether the display mode should be included in enumeration
@@ -593,6 +594,8 @@ void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor, const GLFWgammaramp* ramp)
 GLFWAPI RRCrtc glfwGetX11Adapter(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(None);
     return monitor->x11.crtc;
 }
@@ -600,6 +603,8 @@ GLFWAPI RRCrtc glfwGetX11Adapter(GLFWmonitor* handle)
 GLFWAPI RROutput glfwGetX11Monitor(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
+    assert(monitor != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(None);
     return monitor->x11.output;
 }
diff --git a/glfw/x11_window.c b/glfw/x11_window.c
index 3e25361bb..471b9d8a8 100644
--- a/glfw/x11_window.c
+++ b/glfw/x11_window.c
@@ -41,6 +41,7 @@
 #include <stdlib.h>
 #include <limits.h>
 #include <errno.h>
+#include <assert.h>
 
 // Action for EWMH client messages
 #define _NET_WM_STATE_REMOVE        0
@@ -3408,6 +3409,8 @@ GLFWAPI Display* glfwGetX11Display(void)
 GLFWAPI unsigned long glfwGetX11Window(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(None);
     return window->x11.handle;
 }