diff --git a/src/compress.cpp b/src/compress.cpp
index cba4ba86..8152c4d6 100644
--- a/src/compress.cpp
+++ b/src/compress.cpp
@@ -87,6 +87,8 @@ int upx_compress           ( const upx_bytep src, unsigned  src_len,
     // set available bytes in dst
     if (*dst_len == 0)
         *dst_len = MemBuffer::getSizeForCompression(src_len);
+    // for UPX, we always need a resonably sized outbut buffer
+    assert(*dst_len >= MemBuffer::getSizeForCompression(src_len));
 
     if (!result)
         result = &result_buffer;