diff --git a/modes.cpp b/modes.cpp
index 59cde25cd..8e4428e34 100644
--- a/modes.cpp
+++ b/modes.cpp
@@ -81,6 +81,8 @@ void CFB_ModePolicy::CipherResynchronize(const byte *iv, size_t length)
 
 void CFB_ModePolicy::SetFeedbackSize(unsigned int feedbackSize)
 {
+	CRYPTOPP_ASSERT(feedbackSize >= 0);
+	CRYPTOPP_ASSERT(feedbackSize <= BlockSize());
 	if (feedbackSize > BlockSize())
 		throw InvalidArgument("CFB_Mode: invalid feedback size");
 	m_feedbackSize = feedbackSize ? feedbackSize : BlockSize();