diff --git a/ChangeLog b/ChangeLog
index 82460769..4efea2b3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,7 @@
 - vips_sink_screen() keeps a ref to the input image ... stops a rare race
 - fix a minor accidental ABI break in 8.6.0 -> 8.6.1 [remicollet]
 - fix read of plane-separate TIFFs with large strips [remicollet]
+- fix a C++ warning in composite.cpp [lovell]
 
 10/12/17 started 8.6.1
 - fix mmap window new/free cycling 
diff --git a/libvips/conversion/composite.cpp b/libvips/conversion/composite.cpp
index 335a821b..870aa95b 100644
--- a/libvips/conversion/composite.cpp
+++ b/libvips/conversion/composite.cpp
@@ -980,7 +980,7 @@ vips_composite_base_build( VipsObject *object )
 		if( !vips_image_hasalpha( in[i] ) ) {
 			VipsImage *x;
 
-			if( vips_addalpha( in[i], &x, NULL ) )
+			if( vips_addalpha( in[i], &x, (void *) NULL ) )
 				return( -1 );
 			g_object_unref( in[i] );
 			in[i] = x;
@@ -1032,7 +1032,7 @@ vips_composite_base_build( VipsObject *object )
 		vips_object_local_array( object, composite->n );
 	for( int i = 0; i < composite->n; i++ )
 		if( vips_colourspace( in[i], &compositing[i],
-			composite->compositing_space, NULL ) )
+			composite->compositing_space, (void *) NULL ) )
 			return( -1 );
 	in = compositing;