diff --git a/python/GafferSceneTest/IECoreScenePreviewTest/PrimitiveAlgoTest.py b/python/GafferSceneTest/IECoreScenePreviewTest/PrimitiveAlgoTest.py
index 539cff0c85..ef84021a33 100644
--- a/python/GafferSceneTest/IECoreScenePreviewTest/PrimitiveAlgoTest.py
+++ b/python/GafferSceneTest/IECoreScenePreviewTest/PrimitiveAlgoTest.py
@@ -215,7 +215,22 @@ def testMergePrimitivesSimpleMeshes( self ) :
 			IECore.V3fData( imath.V3f( 0.1, 0.2, 0.3 ), IECore.GeometricData.Interpretation.Point )
 		)
 
+		with IECore.CapturingMessageHandler() as mh:
+			mergedWithMissingN = PrimitiveAlgo.mergePrimitives( [( mesh1, imath.M44f() ), ( mesh2, imath.M44f() ) ] )
+		self.assertEqual( len( mh.messages ), 1 )
+		self.assertEqual( mh.messages[0].context, "mergePrimitives" )
+		self.assertEqual( mh.messages[0].message, 'Primitive variable N missing on some input primitives, defaulting to zero length normals.' )
+
+		# The effect of not specifying a normal is the same as a Constant, zero normal
+		mesh2["N"] = IECoreScene.PrimitiveVariable( Interpolation.Constant,
+			IECore.V3fData( imath.V3f( 0 ), IECore.GeometricData.Interpretation.Normal )
+		)
+
 		merged = PrimitiveAlgo.mergePrimitives( [( mesh1, imath.M44f() ), ( mesh2, imath.M44f() ) ] )
+		print( merged["N"], "\n\n", mergedWithMissingN["N"] )
+		self.assertEqual( merged["N"], mergedWithMissingN["N"] )
+		self.assertEqual( merged, mergedWithMissingN )
+
 		self.assertTrue( merged.arePrimitiveVariablesValid() )
 		self.assertEqual( merged.interpolation, "linear" )
 		self.assertEqual( merged.verticesPerFace, IECore.IntVectorData( [ 4, 3 ] ) )
@@ -436,8 +451,6 @@ def randomC():
 			self.assertEqual( len( mh.messages ), numDiscarded )
 
 			for k in allInterpsRef.keys():
-				#if k == "P":
-					#continue
 
 				kInterp = allInterps[k].interpolation
 
@@ -621,7 +634,13 @@ def testMergeMeshes( self ) :
 		self.resamplePrimVars( referencePlane, [ "altUv", "unindexedFaceVarying" ], Interpolation.FaceVarying )
 
 		# Merge 3 meshes - complex mesh, shifted complex mesh, and a plane
-		merged = PrimitiveAlgo.mergePrimitives( [( source, imath.M44f() ), ( sourcePlane, imath.M44f() ), ( sourceShifted, imath.M44f() )] )
+
+		with IECore.CapturingMessageHandler() as mh:
+			merged = PrimitiveAlgo.mergePrimitives(
+				[( source, imath.M44f() ), ( sourcePlane, imath.M44f() ), ( sourceShifted, imath.M44f() )]
+			)
+		self.assertEqual( len( mh.messages ), 1 )
+		self.assertEqual( mh.messages[0].message, 'Primitive variable N missing on some input primitives, defaulting to zero length normals.' )
 
 		referenceSource["N"] = IECoreScene.PrimitiveVariable( Interpolation.Vertex, IECore.V3fVectorData( [imath.V3f( 0 ) ] * referenceSource.variableSize( Interpolation.Vertex ) ) )
 		referenceShifted["N"] = IECoreScene.PrimitiveVariable( Interpolation.Vertex, IECore.V3fVectorData( [imath.V3f( 0 ) ] * referenceSource.variableSize( Interpolation.Vertex ) ) )
diff --git a/src/GafferScene/IECoreScenePreview/PrimitiveAlgo.cpp b/src/GafferScene/IECoreScenePreview/PrimitiveAlgo.cpp
index 405c8406f0..8fe4f67355 100644
--- a/src/GafferScene/IECoreScenePreview/PrimitiveAlgo.cpp
+++ b/src/GafferScene/IECoreScenePreview/PrimitiveAlgo.cpp
@@ -1029,6 +1029,8 @@ IECoreScene::PrimitivePtr mergePrimitivesInternal(
 
 
 		// We also need to count the amount of data for this primvar contributed by each primitive.
+
+		bool incomplete = false;
 		for( unsigned int i = 0; i < primitives.size(); i++ )
 		{
 
@@ -1046,6 +1048,7 @@ IECoreScene::PrimitivePtr mergePrimitivesInternal(
 				// the indexed case.
 				varInfo.numData[i] = 1;
 				varInfo.indexed = true;
+				incomplete = true;
 				continue;
 			}
 
@@ -1058,6 +1061,18 @@ IECoreScene::PrimitivePtr mergePrimitivesInternal(
 				varInfo.indexed = true;
 			}
 		}
+
+		if( incomplete )
+		{
+			if( name == "N" )
+			{
+				// Using default initialized normals is particularly likely to produce confusion, so we have a special
+				// warning for this case.
+				msg( Msg::Warning, "mergePrimitives",
+					"Primitive variable N missing on some input primitives, defaulting to zero length normals."
+				);
+			}
+		}
 	}
 
 	//