diff --git a/source/blender/render/intern/source/rendercore.c b/source/blender/render/intern/source/rendercore.c
index e8466d603a2..cab825888a1 100644
--- a/source/blender/render/intern/source/rendercore.c
+++ b/source/blender/render/intern/source/rendercore.c
@@ -2282,7 +2282,13 @@ static void shade_tface(BakeShade *bs)
 	
 	if (bs->usemask) {
 		if (bs->ibuf->userdata==NULL) {
-			bs->ibuf->userdata = (void *)MEM_callocN(sizeof(char)*bs->rectx*bs->recty, "BakeMask");
+			BLI_lock_thread(LOCK_CUSTOM1);
+			if (bs->ibuf->userdata==NULL) { /* since the thread was locked, its possible another thread alloced the value */
+				bs->ibuf->userdata = (void *)MEM_callocN(sizeof(char)*bs->rectx*bs->recty, "BakeMask");
+				bs->rect_mask= (char *)bs->ibuf->userdata;
+			}
+			BLI_unlock_thread(LOCK_CUSTOM1);
+		} else {
 			bs->rect_mask= (char *)bs->ibuf->userdata;
 		}
 	}