diff --git a/MobileGL/MG_Impl/GLImpl/Getter/GL_Getter.cpp b/MobileGL/MG_Impl/GLImpl/Getter/GL_Getter.cpp
index aacb58e8..92b03100 100644
--- a/MobileGL/MG_Impl/GLImpl/Getter/GL_Getter.cpp
+++ b/MobileGL/MG_Impl/GLImpl/Getter/GL_Getter.cpp
@@ -1,4 +1,5 @@
 #include "GL_Getter.h"
+#include "MG_State/GLState/ErrorState/ErrorInfo.h"
 #include <Config.h>
 #include <MG_State/GLState/Core.h>
 #include <MG_Util/Converters/GLToStr/GLEnumConverter.h>
@@ -222,9 +223,11 @@ namespace MobileGL {
             case GL_CULL_FACE:
                 *params = MG_State::pGLContext->IsCapabilityEnabled(CapabilityInput::CullFace) ? GL_TRUE : GL_FALSE;
                 break;
-            case GL_CURRENT_PROGRAM:
-                *params = MG_State::pGLContext->GetCurrentProgram()->GetExternalIndex();
+            case GL_CURRENT_PROGRAM: {
+                const auto& currentProgram = MG_State::pGLContext->GetCurrentProgram();
+                *params = currentProgram ? currentProgram->GetExternalIndex() : 0;
                 break;
+            }
             case GL_DEPTH_CLEAR_VALUE:
                 *params = MG_State::pGLContext->GetClearDepth();
                 break;
@@ -253,21 +256,25 @@ namespace MobileGL {
                 *params = 0; // TODO
                 break;
             case GL_DRAW_FRAMEBUFFER_BINDING: {
-                *params = MG_State::pGLContext->GetFramebufferBindingSlot(FramebufferTarget::Draw)
-                              .GetBoundObject()
-                              ->GetExternalIndex();
+                const auto& FBO =
+                    MG_State::pGLContext->GetFramebufferBindingSlot(FramebufferTarget::Draw).GetBoundObject();
+                *params = FBO ? FBO->GetExternalIndex() : 0;
                 break;
             }
             case GL_READ_FRAMEBUFFER_BINDING: {
-                *params = MG_State::pGLContext->GetFramebufferBindingSlot(FramebufferTarget::Read)
-                              .GetBoundObject()
-                              ->GetExternalIndex();
+                const auto& FBO =
+                    MG_State::pGLContext->GetFramebufferBindingSlot(FramebufferTarget::Read).GetBoundObject();
+                *params = FBO ? FBO->GetExternalIndex() : 0;
                 break;
             }
             case GL_ELEMENT_ARRAY_BUFFER_BINDING: {
-                *params = MG_State::pGLContext->GetBufferBindingSlot(BufferTarget::Index)
-                              .GetBoundObject()
-                              ->GetExternalIndex();
+                if (!MG_State::pGLContext->GetBoundVertexArray()) {
+                    *params = 0;
+                    break;
+                }
+                const auto& bufferObject =
+                    MG_State::pGLContext->GetBufferBindingSlot(BufferTarget::Index).GetBoundObject();
+                *params = bufferObject ? bufferObject->GetExternalIndex() : 0;
                 break;
             }
             case GL_FRAGMENT_SHADER_DERIVATIVE_HINT:
@@ -614,8 +621,8 @@ namespace MobileGL {
                 break;
             case GL_SAMPLER_BINDING: {
                 Int unit = MG_State::pGLContext->GetActiveTextureUnit();
-                auto& tu = MG_State::pGLContext->GetTextureUnitObject(unit);
-                auto sampler = tu.GetSamplerObject();
+                const auto& tu = MG_State::pGLContext->GetTextureUnitObject(unit);
+                const auto& sampler = tu.GetSamplerObject();
                 *params = sampler ? static_cast<GLint>(sampler->GetExternalIndex()) : 0;
                 break;
             }
@@ -712,8 +719,8 @@ namespace MobileGL {
             case GL_TEXTURE_BINDING_2D: {
                 Int unit = MG_State::pGLContext->GetActiveTextureUnit();
                 auto& tu = MG_State::pGLContext->GetTextureUnitObject(unit);
-                auto& slot = tu.GetBindingSlot(TextureTarget::Texture2D);
-                auto obj = slot.GetBoundObject();
+                const auto& slot = tu.GetBindingSlot(TextureTarget::Texture2D);
+                const auto& obj = slot.GetBoundObject();
                 *params = obj ? static_cast<GLint>(obj->GetExternalIndex()) : 0;
                 break;
             }
@@ -729,8 +736,8 @@ namespace MobileGL {
             case GL_TEXTURE_BINDING_3D: {
                 Int unit = MG_State::pGLContext->GetActiveTextureUnit();
                 auto& tu = MG_State::pGLContext->GetTextureUnitObject(unit);
-                auto& slot = tu.GetBindingSlot(TextureTarget::Texture3D);
-                auto obj = slot.GetBoundObject();
+                const auto& slot = tu.GetBindingSlot(TextureTarget::Texture3D);
+                const auto& obj = slot.GetBoundObject();
                 *params = obj ? static_cast<GLint>(obj->GetExternalIndex()) : 0;
                 break;
             }
@@ -740,8 +747,8 @@ namespace MobileGL {
             case GL_TEXTURE_BINDING_CUBE_MAP: {
                 Int unit = MG_State::pGLContext->GetActiveTextureUnit();
                 auto& tu = MG_State::pGLContext->GetTextureUnitObject(unit);
-                auto& slot = tu.GetBindingSlot(TextureTarget::TextureCubeMap);
-                auto obj = slot.GetBoundObject();
+                const auto& slot = tu.GetBindingSlot(TextureTarget::TextureCubeMap);
+                const auto& obj = slot.GetBoundObject();
                 *params = obj ? static_cast<GLint>(obj->GetExternalIndex()) : 0;
                 break;
             }
@@ -803,7 +810,7 @@ namespace MobileGL {
                 *params = MG_State::pGLContext->GetPixelStoreParam(PixelStoreParam::UnpackSwapBytes);
                 break;
             case GL_VERTEX_ARRAY_BINDING: {
-                auto vao = MG_State::pGLContext->GetBoundVertexArray();
+                const auto& vao = MG_State::pGLContext->GetBoundVertexArray();
                 *params = vao ? static_cast<GLint>(vao->GetExternalIndex()) : 0;
                 break;
             }
@@ -823,7 +830,7 @@ namespace MobileGL {
                 *params = 0; // TODO
                 break;
             case GL_VIEWPORT: {
-                auto vp = MG_State::pGLContext->GetViewport();
+                const auto& vp = MG_State::pGLContext->GetViewport();
                 params[0] = vp.x();
                 params[1] = vp.y();
                 params[2] = vp.z();
@@ -844,7 +851,10 @@ namespace MobileGL {
                 break;
             default:
                 MGLOG_E("glGetIntegerv: Invalid enum %s (0x%X)", MG_Util::ConvertGLEnumToString(pname).c_str(), pname);
-                // TODO: report GL_INVALID_ENUM.
+                MG_State::pGLContext->RecordError(
+                    ErrorCode::InvalidEnum, MakeShared<GenericErrorInfo>("MG_Impl/GLImpl", "GetIntegerv",
+                                                                         std::format("Invalid enum: 0x{:X}", pname)));
+
                 break;
             }
         }
diff --git a/MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp b/MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp
index 2b072e74..51d34e21 100644
--- a/MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp
+++ b/MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp
@@ -443,6 +443,8 @@ namespace MobileGL {
         }
 
         void UseProgram_State(GLint program) {
+            MGLOG_D("UseProgram_State: program=%u", program);
+
             if (program == 0) {
                 MG_State::pGLContext->UseProgram(0);
                 return;
diff --git a/MobileGL/MG_State/GLState/Core.cpp b/MobileGL/MG_State/GLState/Core.cpp
index 991639f5..ac04c5ac 100644
--- a/MobileGL/MG_State/GLState/Core.cpp
+++ b/MobileGL/MG_State/GLState/Core.cpp
@@ -52,7 +52,14 @@ namespace MobileGL {
 
             BindingSlot<BufferObject>& GLContext::GetBufferBindingSlot(BufferTarget target) {
                 if (target == BufferTarget::Index) {
-                    return m_vertexArrayState.GetBoundVertexArray()->GetIndexBufferBindingSlot();
+                    const auto& vao = m_vertexArrayState.GetBoundVertexArray();
+                    if (vao) {
+                        return vao->GetIndexBufferBindingSlot();
+                    } else {
+                        assert(false);
+                        static BindingSlot<BufferObject> defaultSlot(BufferTarget::Index);
+                        return defaultSlot;
+                    }
                 }
 
                 return m_bufferState.GetBindingSlot(target);
diff --git a/MobileGL/MG_State/GLState/ProgramState/ProgramObject.h b/MobileGL/MG_State/GLState/ProgramState/ProgramObject.h
index 3b1426f4..a7336ee5 100644
--- a/MobileGL/MG_State/GLState/ProgramState/ProgramObject.h
+++ b/MobileGL/MG_State/GLState/ProgramState/ProgramObject.h
@@ -9,7 +9,7 @@ namespace MobileGL {
         namespace GLState {
             class ProgramObject {
             public:
-                ProgramObject(const Uint externalIndex) : m_externalIndex(externalIndex) {}
+                ProgramObject(Uint externalIndex) : m_externalIndex(externalIndex) {}
                 bool ShaderIsAttached(SharedPtr<ShaderObject> shader);
                 bool AttachShader(SharedPtr<ShaderObject> shader);
                 SizeT DetachShader(SharedPtr<ShaderObject> shader);
diff --git a/MobileGL/MG_State/GLState/ProgramState/ShaderObject.h b/MobileGL/MG_State/GLState/ProgramState/ShaderObject.h
index 0e26f425..809a956f 100644
--- a/MobileGL/MG_State/GLState/ProgramState/ShaderObject.h
+++ b/MobileGL/MG_State/GLState/ProgramState/ShaderObject.h
@@ -16,7 +16,7 @@ namespace MobileGL {
         namespace GLState {
             class ShaderObject {
             public:
-                ShaderObject(const ShaderStage stage, const Uint externalIndex)
+                ShaderObject(const ShaderStage stage, Uint externalIndex)
                     : m_stage(stage), m_externalIndex(externalIndex) {}
                 void SetShaderSource(const String& source);
                 void SetShaderSource(String&& source);