diff --git a/libsmtutil/CVC4Interface.cpp b/libsmtutil/CVC4Interface.cpp
index 87184714f..7735d5a16 100644
--- a/libsmtutil/CVC4Interface.cpp
+++ b/libsmtutil/CVC4Interface.cpp
@@ -246,7 +246,7 @@ CVC4::Expr CVC4Interface::toCVC4Expr(Expression const& _expr)
 			smtAssert(tupleSort, "");
 			CVC4::DatatypeType tt = m_context.mkTupleType(cvc4Sort(tupleSort->components));
 			CVC4::Datatype const& dt = tt.getDatatype();
-			size_t index = std::stoi(_expr.arguments[1].name);
+			size_t index = std::stoul(_expr.arguments[1].name);
 			CVC4::Expr s = dt[0][index].getSelector();
 			return m_context.mkExpr(CVC4::kind::APPLY_SELECTOR, s, arguments[0]);
 		}
diff --git a/libsmtutil/SMTLib2Interface.cpp b/libsmtutil/SMTLib2Interface.cpp
index bc37ffb6c..ce1efb5f6 100644
--- a/libsmtutil/SMTLib2Interface.cpp
+++ b/libsmtutil/SMTLib2Interface.cpp
@@ -184,7 +184,7 @@ string SMTLib2Interface::toSExpr(Expression const& _expr)
 	{
 		smtAssert(_expr.arguments.size() == 2, "");
 		auto tupleSort = dynamic_pointer_cast<TupleSort>(_expr.arguments.at(0).sort);
-		unsigned index = std::stoi(_expr.arguments.at(1).name);
+		size_t index = std::stoul(_expr.arguments.at(1).name);
 		smtAssert(index < tupleSort->members.size(), "");
 		sexpr += "|" + tupleSort->members.at(index) + "| " + toSExpr(_expr.arguments.at(0));
 	}
diff --git a/libsmtutil/Z3Interface.cpp b/libsmtutil/Z3Interface.cpp
index 1337b7b20..a71d008ab 100644
--- a/libsmtutil/Z3Interface.cpp
+++ b/libsmtutil/Z3Interface.cpp
@@ -207,7 +207,7 @@ z3::expr Z3Interface::toZ3Expr(Expression const& _expr)
 		}
 		else if (n == "tuple_get")
 		{
-			size_t index = std::stoi(_expr.arguments[1].name);
+			size_t index = stoul(_expr.arguments[1].name);
 			return z3::func_decl(m_context, Z3_get_tuple_sort_field_decl(m_context, z3Sort(*_expr.arguments[0].sort), index))(arguments[0]);
 		}
 		else if (n == "tuple_constructor")