diff --git a/libvips/iofuncs/system.c b/libvips/iofuncs/system.c
index 1082ec1c..335c5f8c 100644
--- a/libvips/iofuncs/system.c
+++ b/libvips/iofuncs/system.c
@@ -206,6 +206,7 @@ vips_system_class_init( VipsSystemClass *class )
 {
 	GObjectClass *gobject_class = G_OBJECT_CLASS( class );
 	VipsObjectClass *vobject_class = VIPS_OBJECT_CLASS( class );
+	VipsOperationClass *operation_class = VIPS_OPERATION_CLASS( class );
 
 	gobject_class->dispose = vips_system_dispose;
 	gobject_class->set_property = vips_object_set_property;
@@ -215,6 +216,10 @@ vips_system_class_init( VipsSystemClass *class )
 	vobject_class->description = _( "run an external command" );
 	vobject_class->build = vips_system_build;
 
+	/* Commands can have side-effects, so don't cache them. 
+	 */
+	operation_class->flags = VIPS_OPERATION_NOCACHE;
+
 	VIPS_ARG_BOXED( class, "in", 0, 
 		_( "Input" ), 
 		_( "Array of input images" ),