diff --git a/builtin-write-tree.c b/builtin-write-tree.c
index 9d640508dd..3a24ce8157 100644
--- a/builtin-write-tree.c
+++ b/builtin-write-tree.c
@@ -13,7 +13,7 @@ static const char write_tree_usage[] =
 
 int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 {
-	int missing_ok = 0, ret;
+	int flags = 0, ret;
 	const char *prefix = NULL;
 	unsigned char sha1[20];
 	const char *me = "git-write-tree";
@@ -22,9 +22,15 @@ int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 	while (1 < argc) {
 		const char *arg = argv[1];
 		if (!strcmp(arg, "--missing-ok"))
-			missing_ok = 1;
+			flags |= WRITE_TREE_MISSING_OK;
 		else if (!prefixcmp(arg, "--prefix="))
 			prefix = arg + 9;
+		else if (!prefixcmp(arg, "--ignore-cache-tree"))
+			/*
+			 * This is only useful for debugging, so I
+			 * do not bother documenting it.
+			 */
+			flags |= WRITE_TREE_IGNORE_CACHE_TREE;
 		else
 			usage(write_tree_usage);
 		argc--; argv++;
@@ -33,7 +39,7 @@ int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 	if (argc > 2)
 		die("too many options");
 
-	ret = write_cache_as_tree(sha1, missing_ok, prefix);
+	ret = write_cache_as_tree(sha1, flags, prefix);
 	switch (ret) {
 	case 0:
 		printf("%s\n", sha1_to_hex(sha1));
diff --git a/cache-tree.c b/cache-tree.c
index 37bf35e636..6dd8411945 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -538,28 +538,32 @@ static struct cache_tree *cache_tree_find(struct cache_tree *it, const char *pat
 	return it;
 }
 
-int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix)
+int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
 {
 	int entries, was_valid, newfd;
+	struct lock_file *lock_file;
 
 	/*
 	 * We can't free this memory, it becomes part of a linked list
 	 * parsed atexit()
 	 */
-	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+	lock_file = xcalloc(1, sizeof(struct lock_file));
 
 	newfd = hold_locked_index(lock_file, 1);
 
 	entries = read_cache();
 	if (entries < 0)
 		return WRITE_TREE_UNREADABLE_INDEX;
+	if (flags & WRITE_TREE_IGNORE_CACHE_TREE)
+		cache_tree_free(&(active_cache_tree));
 
 	if (!active_cache_tree)
 		active_cache_tree = cache_tree();
 
 	was_valid = cache_tree_fully_valid(active_cache_tree);
-
 	if (!was_valid) {
+		int missing_ok = flags & WRITE_TREE_MISSING_OK;
+
 		if (cache_tree_update(active_cache_tree,
 				      active_cache, active_nr,
 				      missing_ok, 0) < 0)
diff --git a/cache-tree.h b/cache-tree.h
index e958835236..eadcad8adc 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -30,11 +30,16 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size);
 int cache_tree_fully_valid(struct cache_tree *);
 int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int);
 
+/* bitmasks to write_cache_as_tree flags */
+#define WRITE_TREE_MISSING_OK 1
+#define WRITE_TREE_IGNORE_CACHE_TREE 2
+
+/* error return codes */
 #define WRITE_TREE_UNREADABLE_INDEX (-1)
 #define WRITE_TREE_UNMERGED_INDEX (-2)
 #define WRITE_TREE_PREFIX_ERROR (-3)
 
-int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix);
+int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix);
 void prime_cache_tree(struct cache_tree **, struct tree *);
 
 #endif