diff --git a/TODO b/TODO
index f430cb0f..3614a9dc 100644
--- a/TODO
+++ b/TODO
@@ -15,7 +15,7 @@
 
 		if( !(op = vips_call( "add", in1, in2 )) )
 			return( -1 );
-	    g_object_get( G_OBJECT( op ), "out", &x );
+		g_object_get( G_OBJECT( op ), "out", &x );
 		if( im_copy( x, out ) ) {
 			g_object_unref( op );
 			return( -1 );
@@ -34,12 +34,12 @@
 
 		if( !(op = vips_call( "add", in1, in2 )) )
 			return( -1 );
-	    g_object_get( G_OBJECT( op ), "out", &out );
+		g_object_get( G_OBJECT( op ), "out", &out );
 
-        // do we need to ref out? or unref it?
+		// do we need to ref out? or unref it?
 
-        // then this unref should leave the single ref on out the only thing 
-        // that keeps the add alive, hopefully
+		// then this unref should leave the single ref on out the 
+		// only thing that keeps the add alive, hopefully
 		g_object_unref( op );
 
 		return( out );
@@ -55,6 +55,28 @@
 
 
 
+  argh how about on build failure, walk the args and destroy all the values
+  in case any of them are holding refs to the failed op, then unref the op
+
+  need to check exactly when objects get reffed as they are copied out of
+  their GValues
+
+
+
+
+  could we support this API still?
+
+  	vips_call( "add", in1, in2, &out )
+
+  walk the valist setting inputs and skipping outputs (they will always be
+  pointers I guess)
+
+  call build(), bail out on failure destroying unsed outputs as above ^^
+
+  walk valist again, unboxing and writing to the output pointers
+
+  now unref op, and op will be unreffed when all outputs are destroyed
+
 
 
 
diff --git a/libvips/arithmetic/arithmetic.c b/libvips/arithmetic/arithmetic.c
index 8c64e97b..e501d05b 100644
--- a/libvips/arithmetic/arithmetic.c
+++ b/libvips/arithmetic/arithmetic.c
@@ -72,19 +72,19 @@ static int
 vips_arithmetic_build( VipsObject *object )
 {
 	VipsArithmetic *arithmetic = VIPS_ARITHMETIC( object );
-    VipsImage *out;
+	VipsImage *out;
 
 	if( VIPS_OBJECT_CLASS( vips_arithmetic_parent_class )->build( object ) )
 		return( -1 );
 
-    if( !(out = vips_image_new()) )
+	if( !(out = vips_image_new()) )
 		return( -1 );
-    g_object_set( arithmetic, "out", out, NULL );
+	g_object_set( arithmetic, "out", out, NULL );
 
 	/* Should we _generate() here? We should keep the params in the object
 	 * ready to be dropped in.
 	 *
-	 * At the moment we _generate() in binary.c and unary.c.
+	 * At the moment we _generate() separately in binary.c and unary.c.
 	 */
 
 #ifdef DEBUG
diff --git a/libvips/include/vips/object.h b/libvips/include/vips/object.h
index eff61483..ff1193e3 100644
--- a/libvips/include/vips/object.h
+++ b/libvips/include/vips/object.h
@@ -83,9 +83,7 @@ VIPS_ARGUMENT_REQUIRED_INPUT 	Eg. the "left" argument for an add operation
 
 VIPS_ARGUMENT_OPTIONAL_INPUT 	Eg. the "caption" for an object
 
-VIPS_ARGUMENT_REQURED_OUTPUT 	Eg. the "result" of an add operation
-
-VIPS_ARGUMENT_OPTIONAL_OUTPUT 	Eg. the "width" of an image
+VIPS_ARGUMENT_OUTPUT 	        Eg. the "result" of an add operation
 
    Other combinations are used internally, eg. supplying the cast-table for an 
    arithmetic operation
@@ -98,19 +96,14 @@ VIPS_ARGUMENT_OPTIONAL_OUTPUT 	Eg. the "width" of an image
 	 VIPS_ARGUMENT_CONSTRUCT | \
 	 VIPS_ARGUMENT_SET_ONCE)
 
-#define VIPS_ARGUMENT_REQUIRED_OUTPUT \
-	(VIPS_ARGUMENT_OUTPUT | \
-	 VIPS_ARGUMENT_REQUIRED | \
-	 VIPS_ARGUMENT_CONSTRUCT | \
-	 VIPS_ARGUMENT_SET_ONCE)
-
 #define VIPS_ARGUMENT_OPTIONAL_INPUT \
 	(VIPS_ARGUMENT_INPUT | \
 	 VIPS_ARGUMENT_CONSTRUCT | \
 	 VIPS_ARGUMENT_SET_ONCE)
 
-#define VIPS_ARGUMENT_OPTIONAL_OUTPUT \
+#define VIPS_ARGUMENT_REQUIRED_OUTPUT \
 	(VIPS_ARGUMENT_OUTPUT | \
+	 VIPS_ARGUMENT_REQUIRED | \
 	 VIPS_ARGUMENT_CONSTRUCT | \
 	 VIPS_ARGUMENT_SET_ONCE)
 
diff --git a/libvips/iofuncs/operation.c b/libvips/iofuncs/operation.c
index 52e28d5b..57a5dc44 100644
--- a/libvips/iofuncs/operation.c
+++ b/libvips/iofuncs/operation.c
@@ -195,8 +195,8 @@ typedef enum {
 } ArgFlags;
 
 static int
-vips_operation_set_valist (VipsOperation * operation,
-				    ArgFlags flags, va_list ap)
+vips_operation_set_valist (VipsOperation * operation, 
+		ArgFlags flags, va_list ap)
 {
   VipsObject *object = VIPS_OBJECT (operation);
   VipsObjectClass *class = VIPS_OBJECT_GET_CLASS (object);
@@ -227,7 +227,8 @@ vips_operation_set_valist (VipsOperation * operation,
 
 	      /* End of stuff copy-pasted from vips_argument_map().
 	       */
-	      if (argument_class->flags & VIPS_ARGUMENT_REQUIRED &&
+	      if ((argument_class->flags & VIPS_ARGUMENT_REQUIRED) &&
+		  (argument_class->flags & VIPS_ARGUMENT_INPUT) &&
 		  !argument_instance->assigned)
 		{
 		  GValue value = { 0 };