diff --git a/TODO b/TODO
index cc09f423..184b4b58 100644
--- a/TODO
+++ b/TODO
@@ -12,7 +12,7 @@
 
 	  easier to get into CPAN or whatever the python thing is called
 
-- [put exif autorotate into jpeg load
+- put exif autorotate into jpeg load
 
   see
 
diff --git a/libvips/include/vips/internal.h b/libvips/include/vips/internal.h
index 317dc1d1..f6d3da7f 100644
--- a/libvips/include/vips/internal.h
+++ b/libvips/include/vips/internal.h
@@ -218,7 +218,7 @@ void vips_draw_operation_init( void );
 void vips_mosaicing_operation_init( void );
 
 guint64 vips__parse_size( const char *size_string );
-int vips__substitute( const char *domain, char *buf, size_t len, char *sub );
+int vips__substitute( char *buf, size_t len, char *sub );
 
 int vips_check_coding_labq( const char *domain, VipsImage *im );
 int vips_check_coding_rad( const char *domain, VipsImage *im );
diff --git a/libvips/iofuncs/system.c b/libvips/iofuncs/system.c
index a5d3d3d4..7ae1459a 100644
--- a/libvips/iofuncs/system.c
+++ b/libvips/iofuncs/system.c
@@ -159,13 +159,19 @@ vips_system_build( VipsObject *object )
 	vips_strncpy( cmd, system->cmd_format, VIPS_PATH_MAX );
 	if( system->in ) 
 		for( i = 0; i < VIPS_AREA( system->in )->n; i++ ) 
-			if( vips__substitute( class->nickname, 
-				cmd, VIPS_PATH_MAX, system->in_name[i] ) )
+			if( vips__substitute( cmd, VIPS_PATH_MAX, 
+				system->in_name[i] ) ) {
+				vips_error( class->nickname, "%s",
+					_( "unable to substitute "
+					"input filename" ) ); 
 				return( -1 ); 
+			}
 	if( system->out_name &&
-		vips__substitute( class->nickname, 
-			cmd, VIPS_PATH_MAX, system->out_name ) )
+		vips__substitute( cmd, VIPS_PATH_MAX, system->out_name ) ) {
+		vips_error( class->nickname, "%s",
+			_( "unable to substitute output filename" ) ); 
 		return( -1 ); 
+	}
 
 	/* Swap all "%%" in the string for a single "%". We need this for
 	 * compatibility with older printf-based vips_system()s which
diff --git a/libvips/iofuncs/util.c b/libvips/iofuncs/util.c
index aab2e106..af287175 100644
--- a/libvips/iofuncs/util.c
+++ b/libvips/iofuncs/util.c
@@ -1596,15 +1596,13 @@ vips_enum_from_nick( const char *domain, GType type, const char *nick )
 	return( -1 );
 }
 
-#define BIGBUF (10000)
-
 /* Scan @buf for the first "%ns" (eg. "%12s") and substitute the 
  * lowest-numbered one for @sub. @buf is @len bytes in size.
  *
  * If there are no %ns, use the first %s.
  */
 int
-vips__substitute( const char *domain, char *buf, size_t len, char *sub )
+vips__substitute( char *buf, size_t len, char *sub )
 {
 	size_t buflen = strlen( buf ); 
 	size_t sublen = strlen( sub ); 
@@ -1646,21 +1644,15 @@ vips__substitute( const char *domain, char *buf, size_t len, char *sub )
 				break;
 			}
 
-	if( !sub_start ) {
-		vips_error( domain, 
-			"%s", _( "string contains no substitute marker" ) ); 
+	if( !sub_start ) 
 		return( -1 ); 
-	}
 
 	before_len = sub_start - buf;
 	marker_len = sub_end - sub_start;
 	after_len = buflen - (before_len + marker_len);
 	final_len = before_len + sublen + after_len + 1;
-	if( final_len > len ) { 
-		vips_error( domain, 
-			"%s", _( "not enough space to substitute" ) ); 
+	if( final_len > len )  
 		return( -1 ); 
-	}
 
 	memmove( buf + before_len + sublen, buf + before_len + marker_len, 
 		after_len + 1 );  
diff --git a/python/test_conversion.py b/python/test_conversion.py
index 2b883b4d..9b19263b 100755
--- a/python/test_conversion.py
+++ b/python/test_conversion.py
@@ -281,19 +281,24 @@ class TestConversion(unittest.TestCase):
         black = self.mono * 0.0
 
         for fmt in all_formats:
-            test = self.colour.bandjoin2(black + max_value[fmt] / 2]).cast(fmt)
+            print fmt
+            test = self.colour.bandjoin2(black + max_value[fmt] / 2.0).cast(fmt)
+            pixel = test.getpoint(30, 30)
+            print 'before', pixel
 
             im = test.flatten()
 
             self.assertEqual(im.bands, 3)
             pixel = im.getpoint(30, 30)
-            self.assertAlmostEqualObjects(pixel, [2, 2, 3])
+            print 'after', pixel
+            self.assertAlmostEqualObjects(pixel, [0, 1, 1])
 
             im = test.flatten(background = [100, 100, 100])
 
             self.assertEqual(im.bands, 3)
             pixel = im.getpoint(30, 30)
-            self.assertAlmostEqualObjects(pixel, [50, 50, 51])
+            print pixel
+            self.assertAlmostEqualObjects(pixel, [50, 51, 51])
             
             
 
diff --git a/tools/vipsthumbnail.c b/tools/vipsthumbnail.c
index 6ebb8e99..4ed8507e 100644
--- a/tools/vipsthumbnail.c
+++ b/tools/vipsthumbnail.c
@@ -75,6 +75,7 @@
 #include <locale.h>
 
 #include <vips/vips.h>
+#include <vips/internal.h>
 
 #define ORIENTATION ("exif-ifd0-Orientation")
 
@@ -699,10 +700,15 @@ thumbnail_write( VipsObject *process, VipsImage *im, const char *filename )
 	if( (p = strrchr( file, '.' )) ) 
 		*p = '\0';
 
+	/* Don't use vips_snprintf(), we only want to optionally substitute a 
+	 * single %s.
+	 */
+	vips_strncpy( buf, output_format, FILENAME_MAX ); 
+	vips__substitute( buf, FILENAME_MAX, file ); 
+
 	/* output_format can be an absolute path, in which case we discard the
 	 * path from the incoming file.
 	 */
-	vips_snprintf( buf, FILENAME_MAX, output_format, file );
 	if( g_path_is_absolute( output_format ) ) 
 		output_name = g_strdup( buf );
 	else {