diff --git a/Changes.md b/Changes.md
index cda5467df69..ceb6d67c4fe 100644
--- a/Changes.md
+++ b/Changes.md
@@ -10,6 +10,9 @@ Fixes
 -----
 
 - RenderMan : Fixed handling of V3f data with non-geometric interpretation. Common sources include `float3` primvars and attributes loaded from USD files.
+- Cycles : 
+  - Fixed potential crashes when using `render:` attributes on objects due to passing invalid data.
+  - Removed passing of `render:` matrix attribute types to objects as they're not supported currently.
 
 Build
 -----
diff --git a/python/GafferCyclesTest/IECoreCyclesPreviewTest/RendererTest.py b/python/GafferCyclesTest/IECoreCyclesPreviewTest/RendererTest.py
index 3783b065a69..7447d2671a8 100644
--- a/python/GafferCyclesTest/IECoreCyclesPreviewTest/RendererTest.py
+++ b/python/GafferCyclesTest/IECoreCyclesPreviewTest/RendererTest.py
@@ -2938,5 +2938,80 @@ def assertVolumesVisible() :
 		del volume2
 		del vdb
 
+	def testCustomAttributes( self ) :
+
+		ignoreWarningMessage = "Custom attribute \"unsupportedMatrix\" has unsupported type \"M44fData\"."
+		self.ignoreMessage( IECore.Msg.Level.Warning, "IECoreCycles::Renderer", ignoreWarningMessage )
+
+		renderer = self.createRenderer()
+
+		renderer.output(
+			"testOutput",
+			IECoreScene.Output(
+				"test",
+				"ieDisplay",
+				"rgba",
+				{
+					"driverType" : "ImageDisplayDriver",
+					"handle" : "testCustomAttributes",
+				}
+			)
+		)
+
+		# Create many float values as this would make Cycles unstable and "glitch" or crash.
+		# Also add invalid ones like matrices to make sure these get skipped with a warning.
+		plane = renderer.object(
+			"/plane",
+			IECoreScene.MeshPrimitive.createPlane(
+				imath.Box2f( imath.V2f( -1 ), imath.V2f( 1 ) ),
+			),
+			renderer.attributes( IECore.CompoundObject ( {
+				"render:displayColor" : IECore.Color3fData( imath.Color3f( 1, 0.5, 0.25 ) ),
+				"render:displayOpacity" : IECore.FloatData( 1.0 ),
+				"render:myFloat" : IECore.FloatData( 0.5 ),
+				"render:anotherFloat" : IECore.FloatData( 0.25 ),
+				"render:myBool0" : IECore.BoolData( 0 ),
+				"render:myBool1" : IECore.BoolData( 1 ),
+				"render:unsupportedMatrix" : IECore.M44fData( imath.M44f() ),
+				"cycles:surface" : IECoreScene.ShaderNetwork(
+					shaders = {
+						"output" : IECoreScene.Shader( "principled_bsdf", "cycles:surface", { "emission_strength" : 1 } ),
+						"combineColor" : IECoreScene.Shader( "combine_color", "cycles:shader" ),
+						"attributeMyFloat" : IECoreScene.Shader( "attribute", "cycles:shader", { "attribute" : IECore.StringData( "myFloat" ) } ),
+						"attributeAnotherFloat" : IECoreScene.Shader( "attribute", "cycles:shader", { "attribute" : IECore.StringData( "anotherFloat" ) } ),
+						"attributeMyBool1" : IECoreScene.Shader( "attribute", "cycles:shader", { "attribute" : IECore.StringData( "myBool1" ) } ),
+					},
+					connections = [
+						( ( "attributeMyFloat", "fac" ), ( "combineColor", "r" ) ),
+						( ( "attributeAnotherFloat", "fac" ), ( "combineColor", "g" ) ),
+						( ( "attributeMyBool1", "fac" ), ( "combineColor", "b" ) ),
+						( ( "combineColor", "color" ), ( "output", "emission_color" ) ),
+					],
+					output = "output",
+				)
+			} ) )
+		)
+		plane.transform( imath.M44f().translate( imath.V3f( 0, 0, -1 ) ) )
+
+		with IECore.CapturingMessageHandler() as mh :
+			renderer.render()
+			for m in mh.messages :
+				if m.level != IECore.MessageHandler.Level.Warning :
+					continue
+				self.assertEqual( m.context, "IECoreCycles::Renderer" )
+				self.assertEqual( m.message, ignoreWarningMessage )
+				break
+
+		image = IECoreImage.ImageDisplayDriver.storedImage( "testCustomAttributes" )
+		self.assertIsInstance( image, IECoreImage.ImagePrimitive )
+
+		# Slightly off-centre, to avoid triangle edge artifact in centre of image.
+		testPixel = self.__colorAtUV( image, imath.V2f( 0.55 ) )
+		self.assertEqual( testPixel.r, 0.5 )
+		self.assertEqual( testPixel.g, 0.25 )
+		self.assertEqual( testPixel.b, 1.0 )
+
+		del plane
+
 if __name__ == "__main__":
 	unittest.main()
diff --git a/src/GafferCycles/IECoreCyclesPreview/Renderer.cpp b/src/GafferCycles/IECoreCyclesPreview/Renderer.cpp
index 6ee574b2a2d..040bb29a1f9 100644
--- a/src/GafferCycles/IECoreCyclesPreview/Renderer.cpp
+++ b/src/GafferCycles/IECoreCyclesPreview/Renderer.cpp
@@ -994,7 +994,7 @@ class CyclesAttributes : public IECoreScenePreview::Renderer::AttributesInterfac
 			for( const auto &attr : customMap )
 			{
 				ccl::ParamValue paramValue = SocketAlgo::setParamValue( attr.first, attr.second.get() );
-				if( paramValue.data() )
+				if( paramValue.nvalues() == 1 )
 				{
 					m_custom.push_back( paramValue );
 				}
diff --git a/src/GafferCycles/IECoreCyclesPreview/SocketAlgo.cpp b/src/GafferCycles/IECoreCyclesPreview/SocketAlgo.cpp
index c725002b636..d418b00adcd 100644
--- a/src/GafferCycles/IECoreCyclesPreview/SocketAlgo.cpp
+++ b/src/GafferCycles/IECoreCyclesPreview/SocketAlgo.cpp
@@ -633,23 +633,11 @@ ccl::ParamValue setParamValue( const IECore::InternedString &name, const IECore:
 				return ccl::ParamValue( name.string(), ccl::TypeFloat4, 1, &result );
 			}
 			break;
-		case M44fDataTypeId :
-			{
-				const M44fData *data = static_cast<const M44fData *>( value );
-				const ccl::Transform result = setTransform( data->readable() );
-				return ccl::ParamValue( name.string(), ccl::TypeMatrix, 1, &result );
-			}
-			break;
-		case M44dDataTypeId :
-			{
-				const M44dData *data = static_cast<const M44dData *>( value );
-				const ccl::Transform result = setTransform( data->readable() );
-				return ccl::ParamValue( name.string(), ccl::TypeMatrix, 1, &result );
-			}
-			break;
 		default :
 			{
-				// A ParamValue that we can test with .data() to see if it's a nullptr.
+				// Don't check with nullptr as OpenImageIO's implementation will store the value
+				// directly if it is small enough and won't be a pointer of null when using
+				// .data(), instead check for .nvalues() > 0 on the return ParamValue struct.
 				return ccl::ParamValue();
 			}
 	}