diff --git a/applypatch/imgpatch.cpp b/applypatch/imgpatch.cpp
index c4c2707..2f8f485 100644
--- a/applypatch/imgpatch.cpp
+++ b/applypatch/imgpatch.cpp
@@ -54,6 +54,7 @@
                                             const Value& patch, size_t patch_offset,
                                             const char* deflate_header, SinkFn sink) {
   size_t expected_target_length = static_cast<size_t>(Read8(deflate_header + 32));
+  CHECK_GT(expected_target_length, 0);
   int level = Read4(deflate_header + 40);
   int method = Read4(deflate_header + 44);
   int window_bits = Read4(deflate_header + 48);