diff --git a/libvips/iofuncs/type.c b/libvips/iofuncs/type.c
index 84ecf351..00fe374d 100644
--- a/libvips/iofuncs/type.c
+++ b/libvips/iofuncs/type.c
@@ -37,9 +37,9 @@
  */
 
 /*
+ */
 #define VIPS_DEBUG
 #define DEBUG
- */
 
 #ifdef HAVE_CONFIG_H
 #include <config.h>
@@ -150,7 +150,7 @@ vips_area_copy( VipsArea *area )
 {
 	g_mutex_lock( area->lock );
 
-	g_assert( area->count >= 0 );
+	g_assert( area->count > 0 );
 
 	area->count += 1;
 
@@ -192,12 +192,17 @@ vips_area_unref( VipsArea *area )
 
 		g_free( area );
 
-		if( vips__leak )
+		if( vips__leak ) {
+			g_mutex_lock( vips__global_lock );
 			vips_area_all = g_slist_remove( vips_area_all, area ); 
+			g_mutex_unlock( vips__global_lock );
+		}
 
 #ifdef DEBUG
+		g_mutex_lock( vips__global_lock );
 		printf( "vips_area_unref: free .. total = %d\n", 
 			g_slist_length( vips_area_all ) );
+		g_mutex_unlock( vips__global_lock );
 #endif /*DEBUG*/
 	}
 	else
@@ -230,13 +235,18 @@ vips_area_new( VipsCallbackFn free_fn, void *data )
 	area->type = 0;
 	area->sizeof_type = 0;
 
-	if( vips__leak )
+	if( vips__leak ) {
+		g_mutex_lock( vips__global_lock );
 		vips_area_all = g_slist_prepend( vips_area_all, area ); 
+		g_mutex_unlock( vips__global_lock );
+	}
 
 #ifdef DEBUG
+	g_mutex_lock( vips__global_lock );
 	printf( "vips_area_new: %p count = %d (%d in total)\n", 
 		area, area->count, 
 		g_slist_length( vips_area_all ) );
+	g_mutex_unlock( vips__global_lock );
 #endif /*DEBUG*/
 
 	return( area );