diff --git a/cache-tree.c b/cache-tree.c
index 345ea35963..34baa6d85a 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -354,7 +354,9 @@ static int update_one(struct cache_tree *it,
 			entlen = pathlen - baselen;
 			i++;
 		}
-		if (mode != S_IFGITLINK && !missing_ok && !has_sha1_file(sha1)) {
+
+		if (is_null_sha1(sha1) ||
+		    (mode != S_IFGITLINK && !missing_ok && !has_sha1_file(sha1))) {
 			strbuf_release(&buffer);
 			if (expected_missing)
 				return -1;
diff --git a/read-cache.c b/read-cache.c
index 9054369dd0..2ca2cae617 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2021,6 +2021,7 @@ static int do_write_index(struct index_state *istate, int newfd,
 	int entries = istate->cache_nr;
 	struct stat st;
 	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
+	int drop_cache_tree = 0;
 
 	for (i = removed = extended = 0; i < entries; i++) {
 		if (cache[i]->ce_flags & CE_REMOVE)
@@ -2071,6 +2072,8 @@ static int do_write_index(struct index_state *istate, int newfd,
 				warning(msg, ce->name);
 			else
 				return error(msg, ce->name);
+
+			drop_cache_tree = 1;
 		}
 		if (ce_write_entry(&c, newfd, ce, previous_name) < 0)
 			return -1;
@@ -2089,7 +2092,7 @@ static int do_write_index(struct index_state *istate, int newfd,
 		if (err)
 			return -1;
 	}
-	if (!strip_extensions && istate->cache_tree) {
+	if (!strip_extensions && !drop_cache_tree && istate->cache_tree) {
 		struct strbuf sb = STRBUF_INIT;
 
 		cache_tree_write(&sb, istate->cache_tree);
diff --git a/t/t1601-index-bogus.sh b/t/t1601-index-bogus.sh
new file mode 100755
index 0000000000..73cc9323cd
--- /dev/null
+++ b/t/t1601-index-bogus.sh
@@ -0,0 +1,22 @@
+#!/bin/sh
+
+test_description='test handling of bogus index entries'
+. ./test-lib.sh
+
+test_expect_success 'create tree with null sha1' '
+	tree=$(printf "160000 commit $_z40\\tbroken\\n" | git mktree)
+'
+
+test_expect_success 'read-tree refuses to read null sha1' '
+	test_must_fail git read-tree $tree
+'
+
+test_expect_success 'GIT_ALLOW_NULL_SHA1 overrides refusal' '
+	GIT_ALLOW_NULL_SHA1=1 git read-tree $tree
+'
+
+test_expect_success 'git write-tree refuses to write null sha1' '
+	test_must_fail git write-tree
+'
+
+test_done
diff --git a/t/t7009-filter-branch-null-sha1.sh b/t/t7009-filter-branch-null-sha1.sh
index c27f90f285..a8d9ec4987 100755
--- a/t/t7009-filter-branch-null-sha1.sh
+++ b/t/t7009-filter-branch-null-sha1.sh
@@ -31,6 +31,12 @@ test_expect_success 'setup: bring HEAD and index in sync' '
 	git commit -a -m "back to normal"
 '
 
+test_expect_success 'noop filter-branch complains' '
+	test_must_fail git filter-branch \
+		--force --prune-empty \
+		--index-filter "true"
+'
+
 test_expect_success 'filter commands are still checked' '
 	test_must_fail git filter-branch \
 		--force --prune-empty \