diff --git a/OSBindings/Mac/Clock Signal/Wrappers/CSAtari2600.mm b/OSBindings/Mac/Clock Signal/Wrappers/CSAtari2600.mm
index de8a51f84..4dab3724c 100644
--- a/OSBindings/Mac/Clock Signal/Wrappers/CSAtari2600.mm	
+++ b/OSBindings/Mac/Clock Signal/Wrappers/CSAtari2600.mm	
@@ -41,7 +41,9 @@ struct CRTDelegate: public Outputs::CRT::Delegate {
 			_didDecideRegion = YES;
 			if(numberOfUnexpectedSyncs >= numberOfFrames >> 1)
 			{
-				_atari2600.switch_region();
+				[self.view performWithGLContext:^{
+					_atari2600.switch_region();
+				}];
 			}
 		}
 	}
diff --git a/Outputs/CRT/Internals/CRTOpenGL.cpp b/Outputs/CRT/Internals/CRTOpenGL.cpp
index 4253df122..5ebbc25d5 100644
--- a/Outputs/CRT/Internals/CRTOpenGL.cpp
+++ b/Outputs/CRT/Internals/CRTOpenGL.cpp
@@ -909,6 +909,7 @@ void OpenGLOutputBuilder::set_timing(unsigned int cycles_per_line, unsigned int
 
 void OpenGLOutputBuilder::set_colour_space_uniforms()
 {
+	_output_mutex->lock();
 	GLfloat rgbToYUV[] = {0.299f, -0.14713f, 0.615f, 0.587f, -0.28886f, -0.51499f, 0.114f, 0.436f, -0.10001f};
 	GLfloat yuvToRGB[] = {1.0f, 1.0f, 1.0f, 0.0f, -0.39465f, 2.03211f, 1.13983f, -0.58060f, 0.0f};
 
@@ -949,10 +950,12 @@ void OpenGLOutputBuilder::set_colour_space_uniforms()
 			glUniformMatrix3fv(uniform, 1, GL_FALSE, toRGB);
 		}
 	}
+	_output_mutex->unlock();
 }
 
 void OpenGLOutputBuilder::set_timing_uniforms()
 {
+	_output_mutex->lock();
 	OpenGL::Shader *intermediate_shaders[] = {
 		composite_input_shader_program.get(),
 		composite_y_filter_shader_program.get(),
@@ -1021,4 +1024,5 @@ void OpenGLOutputBuilder::set_timing_uniforms()
 		chrominance_filter.get_coefficients(weights);
 		glUniform4fv(weightsUniform, 3, weights);
 	}
+	_output_mutex->unlock();
 }