diff --git a/cryptlib.cpp b/cryptlib.cpp
index dadd9cec..df138ddb 100644
--- a/cryptlib.cpp
+++ b/cryptlib.cpp
@@ -87,7 +87,7 @@ void SimpleKeyingInterface::ThrowIfResynchronizable()
 
 void SimpleKeyingInterface::ThrowIfInvalidIV(const byte *iv)
 {
-	if (!iv && !(IVRequirement() == INTERNALLY_GENERATED_IV || IVRequirement() == UNIQUE_IV || !IsResynchronizable()))
+	if (!iv && IVRequirement() == UNPREDICTABLE_RANDOM_IV)
 		throw InvalidArgument(GetAlgorithm().AlgorithmName() + ": this object cannot use a null IV");
 }
 
diff --git a/modes.cpp b/modes.cpp
index 789fafb2..46332284 100644
--- a/modes.cpp
+++ b/modes.cpp
@@ -56,7 +56,8 @@ void CFB_ModePolicy::TransformRegister()
 
 void CFB_ModePolicy::CipherResynchronize(const byte *iv, size_t length)
 {
-	memcpy_s(m_register, m_register.size(), iv, BlockSize());
+	assert(length == BlockSize());
+	CopyOrZero(m_register, iv, length);
 	TransformRegister();
 }
 
@@ -85,6 +86,7 @@ void OFB_ModePolicy::WriteKeystream(byte *keystreamBuffer, size_t iterationCount
 
 void OFB_ModePolicy::CipherResynchronize(byte *keystreamBuffer, const byte *iv, size_t length)
 {
+	assert(length == BlockSize());
 	CopyOrZero(m_register, iv, length);
 }