diff --git a/diff.h b/diff.h
index e79f3b3ff0..5d7b9f7577 100644
--- a/diff.h
+++ b/diff.h
@@ -189,8 +189,6 @@ const char *diff_line_prefix(struct diff_options *);
 
 extern const char mime_boundary_leader[];
 
-extern int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
-		     const char *base, struct diff_options *opt);
 extern int diff_tree_sha1(const unsigned char *old, const unsigned char *new,
 			  const char *base, struct diff_options *opt);
 extern int diff_root_tree_sha1(const unsigned char *new, const char *base,
diff --git a/tree-diff.c b/tree-diff.c
index 8e04002c0e..0e43906b43 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -141,8 +141,8 @@ static void skip_uninteresting(struct tree_desc *t, struct strbuf *base,
 	}
 }
 
-int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
-	      const char *base_str, struct diff_options *opt)
+static int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
+		     const char *base_str, struct diff_options *opt)
 {
 	struct strbuf base;
 	int baselen = strlen(base_str);