diff --git display/simgraph16.cc display/simgraph16.cc
index e55b77065..37777a9a7 100644
--- display/simgraph16.cc
+++ display/simgraph16.cc
@@ -39,6 +39,7 @@
 // currently just redrawing/rezooming
 static pthread_mutex_t rezoom_img_mutex[MAX_THREADS];
 static pthread_mutex_t recode_img_mutex;
+static pthread_mutex_t dirty_img_mutex; //< for marking tile as dirty
 #endif
 
 // to pass the extra clipnum when not needed use this
@@ -1014,6 +1015,8 @@ static void mark_rect_dirty_nc(KOORD_VAL x1, KOORD_VAL y1, KOORD_VAL x2, KOORD_V
 	assert(y2 < tile_lines);
 #endif
 
+	pthread_mutex_lock(&dirty_img_mutex);
+
 	for(  ;  y1 <= y2;  y1++  ) {
 		int bit = y1 * tile_buffer_per_line + x1;
 		const int end = bit + x2 - x1;
@@ -1021,6 +1024,8 @@ static void mark_rect_dirty_nc(KOORD_VAL x1, KOORD_VAL y1, KOORD_VAL x2, KOORD_V
 			tile_dirty[bit >> 5] |= 1 << (bit & 31);
 		} while(  ++bit <= end  );
 	}
+
+	pthread_mutex_unlock(&dirty_img_mutex);
 }
 
 
@@ -5052,6 +5057,7 @@ void simgraph_init(KOORD_VAL width, KOORD_VAL height, int full_screen)
 
 #ifdef MULTI_THREAD
 	pthread_mutex_init( &recode_img_mutex, NULL );
+	pthread_mutex_init( &dirty_img_mutex, NULL );
 #endif
 
 	// init rezoom_img()
@@ -5182,6 +5188,7 @@ void simgraph_exit()
 	tile_dirty = tile_dirty_old = NULL;
 	images = NULL;
 #ifdef MULTI_THREAD
+	pthread_mutex_destroy( &dirty_img_mutex );
 	pthread_mutex_destroy( &recode_img_mutex );
 	for(  int i = 0;  i < MAX_THREADS;  i++  ) {
 		pthread_mutex_destroy( &rezoom_img_mutex[i] );