diff --git a/libyul/optimiser/Solver.cpp b/libyul/optimiser/Solver.cpp
index aae7b22cb..151ee64a2 100644
--- a/libyul/optimiser/Solver.cpp
+++ b/libyul/optimiser/Solver.cpp
@@ -102,7 +102,7 @@ smtutil::Expression Solver::booleanValue(smtutil::Expression _value) const
 	return smtutil::Expression::ite(_value, constantValue(1), constantValue(0));
 }
 
-smtutil::Expression Solver::constantValue(size_t _value) const
+smtutil::Expression Solver::constantValue(bigint _value) const
 {
 	return _value;
 }
diff --git a/libyul/optimiser/Solver.h b/libyul/optimiser/Solver.h
index e9f492901..0f632e32f 100644
--- a/libyul/optimiser/Solver.h
+++ b/libyul/optimiser/Solver.h
@@ -72,7 +72,7 @@ protected:
 
 	virtual std::shared_ptr<smtutil::Sort> defaultSort() const;
 	virtual smtutil::Expression booleanValue(smtutil::Expression _value) const;
-	virtual smtutil::Expression constantValue(size_t _value) const;
+	virtual smtutil::Expression constantValue(bigint _value) const;
 	virtual smtutil::Expression literalValue(Literal const& _literal) const;
 	virtual smtutil::Expression unsignedToSigned(smtutil::Expression _value);
 	virtual smtutil::Expression signedToUnsigned(smtutil::Expression _value);