diff --git a/libevmasm/Assembly.cpp b/libevmasm/Assembly.cpp
index ae8d001d7..88bddd7f7 100644
--- a/libevmasm/Assembly.cpp
+++ b/libevmasm/Assembly.cpp
@@ -519,8 +519,11 @@ map<u256, u256> Assembly::optimiseInternal(
 
 LinkerObject const& Assembly::assemble() const
 {
+	// Return the already assembled object, if present.
 	if (!m_assembledObject.bytecode.empty())
 		return m_assembledObject;
+	// Otherwise ensure the object is actually clear.
+	assertThrow(m_assembledObject.linkReferences.empty(), AssemblyException, "Unexpected link references.");
 
 	size_t subTagSize = 1;
 	for (auto const& sub: m_subs)
diff --git a/libevmasm/LinkerObject.h b/libevmasm/LinkerObject.h
index 928908032..3630365e0 100644
--- a/libevmasm/LinkerObject.h
+++ b/libevmasm/LinkerObject.h
@@ -35,7 +35,9 @@ namespace eth
  */
 struct LinkerObject
 {
+	/// The bytecode.
 	bytes bytecode;
+
 	/// Map from offsets in bytecode to library identifiers. The addresses starting at those offsets
 	/// need to be replaced by the actual addresses by the linker.
 	std::map<size_t, std::string> linkReferences;
@@ -47,7 +49,7 @@ struct LinkerObject
 	void link(std::map<std::string, h160> const& _libraryAddresses);
 
 	/// @returns a hex representation of the bytecode of the given object, replacing unlinked
-	/// addresses by placeholders.
+	/// addresses by placeholders. This output is lowercase.
 	std::string toHex() const;
 
 	/// @returns a 36 character string that is used as a placeholder for the library