diff --git a/Changelog.md b/Changelog.md
index 2f3067771..992de9c11 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -15,6 +15,7 @@ Bugfixes:
  * ABIEncoderV2: Fix internal error related to mappings as library parameters.
  * ABIEncoderV2: Fix invalid signature for events containing structs emitted in libraries.
  * Inline Assembly: Proper error message for missing variables.
+ * Optimizer: Fix internal error related to unused tag removal across assemblies. This never generated any invalid code.
  * SMTChecker: Fixed crash when used with fixed-sized arrays.
  * Yul: Properly detect name clashes with functions before their declaration.
 
diff --git a/libevmasm/Assembly.cpp b/libevmasm/Assembly.cpp
index 7cf7137d2..6b9ece01e 100644
--- a/libevmasm/Assembly.cpp
+++ b/libevmasm/Assembly.cpp
@@ -390,7 +390,7 @@ Assembly& Assembly::optimise(OptimiserSettings const& _settings)
 
 map<u256, u256> Assembly::optimiseInternal(
 	OptimiserSettings const& _settings,
-	std::set<size_t> const& _tagsReferencedFromOutside
+	std::set<size_t> _tagsReferencedFromOutside
 )
 {
 	// Run optimisation for sub-assemblies.
@@ -436,7 +436,22 @@ map<u256, u256> Assembly::optimiseInternal(
 			BlockDeduplicator dedup{m_items};
 			if (dedup.deduplicate())
 			{
-				tagReplacements.insert(dedup.replacedTags().begin(), dedup.replacedTags().end());
+				for (auto const& replacement: dedup.replacedTags())
+				{
+					assertThrow(
+						replacement.first <= size_t(-1) && replacement.second <= size_t(-1),
+						OptimizerException,
+						"Invalid tag replacement."
+					);
+					assertThrow(
+						!tagReplacements.count(replacement.first),
+						OptimizerException,
+						"Replacement already known."
+					);
+					tagReplacements[replacement.first] = replacement.second;
+					if (_tagsReferencedFromOutside.erase(size_t(replacement.first)))
+						_tagsReferencedFromOutside.insert(size_t(replacement.second));
+				}
 				count++;
 			}
 		}
diff --git a/libevmasm/Assembly.h b/libevmasm/Assembly.h
index 1718fbaef..bb5a54b8c 100644
--- a/libevmasm/Assembly.h
+++ b/libevmasm/Assembly.h
@@ -154,7 +154,7 @@ protected:
 	/// Does the same operations as @a optimise, but should only be applied to a sub and
 	/// returns the replaced tags. Also takes an argument containing the tags of this assembly
 	/// that are referenced in a super-assembly.
-	std::map<u256, u256> optimiseInternal(OptimiserSettings const& _settings, std::set<size_t> const& _tagsReferencedFromOutside);
+	std::map<u256, u256> optimiseInternal(OptimiserSettings const& _settings, std::set<size_t> _tagsReferencedFromOutside);
 
 	unsigned bytesRequired(unsigned subTagSize) const;