diff --git a/imgui.cpp b/imgui.cpp
index 564708e1e..2e140e47e 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -3810,6 +3810,7 @@ void ImGui::NewFrame()
     g.WindowsActiveCount = 0;
 
     // Setup current font and draw list
+    g.IO.Fonts->Locked = true;
     SetCurrentFont(GetDefaultFont());
     IM_ASSERT(g.Font->IsLoaded());
     g.DrawListSharedData.ClipRectFullscreen = ImVec4(0.0f, 0.0f, g.IO.DisplaySize.x, g.IO.DisplaySize.y);
@@ -4451,6 +4452,9 @@ void ImGui::EndFrame()
     IM_ASSERT(g.Windows.Size == g.WindowsSortBuffer.Size);  // we done something wrong
     g.Windows.swap(g.WindowsSortBuffer);
 
+    // Unlock font atlas
+    g.IO.Fonts->Locked = false;
+
     // Clear Input data for next frame
     g.IO.MouseWheel = g.IO.MouseWheelH = 0.0f;
     memset(g.IO.InputCharacters, 0, sizeof(g.IO.InputCharacters));
diff --git a/imgui.h b/imgui.h
index f861fb365..0b963c13d 100644
--- a/imgui.h
+++ b/imgui.h
@@ -1852,6 +1852,7 @@ struct ImFontAtlas
     // Members
     //-------------------------------------------
 
+    bool                        Locked;             // Marked as Locked by ImGui::NewFrame() so attempt to modify the atlas will assert.
     ImFontAtlasFlags            Flags;              // Build flags (see ImFontAtlasFlags_)
     ImTextureID                 TexID;              // User data to refer to the texture once it has been uploaded to user's graphic systems. It is passed back to you during rendering via the ImDrawCmd structure.
     int                         TexDesiredWidth;    // Texture width desired by user before Build(). Must be a power-of-two. If have many glyphs your graphics API have texture size restrictions you may want to increase texture width to decrease height.
diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 0f69a82b2..bf5558d45 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -1378,6 +1378,7 @@ static const ImVec2 FONT_ATLAS_DEFAULT_TEX_CURSOR_DATA[ImGuiMouseCursor_COUNT][3
 
 ImFontAtlas::ImFontAtlas()
 {
+    Locked = false;
     Flags = 0x00;
     TexID = NULL;
     TexDesiredWidth = 0;
@@ -1394,11 +1395,13 @@ ImFontAtlas::ImFontAtlas()
 
 ImFontAtlas::~ImFontAtlas()
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     Clear();
 }
 
 void    ImFontAtlas::ClearInputData()
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     for (int i = 0; i < ConfigData.Size; i++)
         if (ConfigData[i].FontData && ConfigData[i].FontDataOwnedByAtlas)
         {
@@ -1421,6 +1424,7 @@ void    ImFontAtlas::ClearInputData()
 
 void    ImFontAtlas::ClearTexData()
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     if (TexPixelsAlpha8)
         ImGui::MemFree(TexPixelsAlpha8);
     if (TexPixelsRGBA32)
@@ -1431,6 +1435,7 @@ void    ImFontAtlas::ClearTexData()
 
 void    ImFontAtlas::ClearFonts()
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     for (int i = 0; i < Fonts.Size; i++)
         IM_DELETE(Fonts[i]);
     Fonts.clear();
@@ -1485,6 +1490,7 @@ void    ImFontAtlas::GetTexDataAsRGBA32(unsigned char** out_pixels, int* out_wid
 
 ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg)
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     IM_ASSERT(font_cfg->FontData != NULL && font_cfg->FontDataSize > 0);
     IM_ASSERT(font_cfg->SizePixels > 0.0f);
 
@@ -1546,6 +1552,7 @@ ImFont* ImFontAtlas::AddFontDefault(const ImFontConfig* font_cfg_template)
 
 ImFont* ImFontAtlas::AddFontFromFileTTF(const char* filename, float size_pixels, const ImFontConfig* font_cfg_template, const ImWchar* glyph_ranges)
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     size_t data_size = 0;
     void* data = ImFileLoadToMemory(filename, "rb", &data_size, 0);
     if (!data)
@@ -1567,6 +1574,7 @@ ImFont* ImFontAtlas::AddFontFromFileTTF(const char* filename, float size_pixels,
 // NB: Transfer ownership of 'ttf_data' to ImFontAtlas, unless font_cfg_template->FontDataOwnedByAtlas == false. Owned TTF buffer will be deleted after Build().
 ImFont* ImFontAtlas::AddFontFromMemoryTTF(void* ttf_data, int ttf_size, float size_pixels, const ImFontConfig* font_cfg_template, const ImWchar* glyph_ranges)
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     ImFontConfig font_cfg = font_cfg_template ? *font_cfg_template : ImFontConfig();
     IM_ASSERT(font_cfg.FontData == NULL);
     font_cfg.FontData = ttf_data;
@@ -1660,6 +1668,7 @@ bool ImFontAtlas::GetMouseCursorTexData(ImGuiMouseCursor cursor_type, ImVec2* ou
 
 bool    ImFontAtlas::Build()
 {
+    IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     return ImFontAtlasBuildWithStbTruetype(this);
 }