diff --git a/diff-lib.c b/diff-lib.c
index ac4e310438..086476bd77 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -508,11 +508,9 @@ static void do_oneway_diff(struct unpack_trees_options *o,
  * For diffing, the index is more important, and we only have a
  * single tree.
  *
- * We're supposed to advance o->pos to skip what we have already processed.
- *
  * This wrapper makes it all more readable, and takes care of all
  * the fairly complex unpack_trees() semantic requirements, including
- * the skipping, the path matching, the type conflict cases etc.
+ * the path matching, the type conflict cases etc.
  */
 static int oneway_diff(const struct cache_entry * const *src,
 		       struct unpack_trees_options *o)
@@ -540,6 +538,11 @@ static int oneway_diff(const struct cache_entry * const *src,
 	if (!idx && !tree)
 		BUG("oneway_diff with neither idx nor tree");
 
+	if (revs->diffopt.prefix &&
+	    strncmp((idx ? idx : tree)->name, revs->diffopt.prefix,
+		    revs->diffopt.prefix_length))
+		return 0;
+
 	if (ce_path_match(revs->diffopt.repo->index,
 			  idx ? idx : tree,
 			  &revs->prune_data, NULL)) {