diff --git a/conandata.yml b/conandata.yml
index dd5ab17..cfb74a8 100644
--- a/conandata.yml
+++ b/conandata.yml
@@ -1 +1 @@
-version: "1.2.0"
+version: "1.2.1"
diff --git a/src/ast/comp_chain_expr.cpp b/src/ast/comp_chain_expr.cpp
index 7304f09..4afefb3 100644
--- a/src/ast/comp_chain_expr.cpp
+++ b/src/ast/comp_chain_expr.cpp
@@ -1,6 +1,7 @@
 #include "cura-formulae-engine/ast/comp_chain_expr.h"
 #include "cura-formulae-engine/eval.h"
 
+#include <algorithm>
 #include <string>
 #include <unordered_set>
 #include <variant>
@@ -90,30 +91,31 @@ namespace CuraFormulaeEngine::ast
             break;
         case Member:
         case NotMember:
-            if (!right_value_result.has_value())
+        {
+            bool found = false;
+            if (std::holds_alternative<std::string>(right_value.value))
             {
-                return zeus::unexpected(right_value_result.error());
+                if (!std::holds_alternative<std::string>(left_value.value))
+                {
+                    return zeus::unexpected(eval::Error::TypeMismatch);
+                }
+                const auto& haystack = std::get<std::string>(right_value.value);
+                const auto& needle = std::get<std::string>(left_value.value);
+                found = haystack.find(needle) != std::string::npos;
             }
-
-            const auto& rhs_value = right_value_result.value();
-            if (!std::holds_alternative<std::vector<eval::Value>>(rhs_value.value))
+            else if (std::holds_alternative<std::vector<eval::Value>>(right_value.value))
             {
-                return zeus::unexpected(eval::Error::TypeMismatch);
+                const auto& list = std::get<std::vector<eval::Value>>(right_value.value);
+                found = std::ranges::any_of(list, [&](const eval::Value& item) { return left_value == item; });
             }
-
-            comparison_result = operators[i] == NotMember;
-
-            const auto& list = std::get<std::vector<eval::Value>>(rhs_value.value);
-            for (const auto& item : list)
+            else
             {
-                if (left_value == item)
-                {
-                    comparison_result = operators[i] == Member;
-                    break;
-                }
+                return zeus::unexpected(eval::Error::TypeMismatch);
             }
+            comparison_result = (operators[i] == Member) ? found : !found;
             break;
         }
+        }
 
         if (!comparison_result.has_value())
         {
diff --git a/tests/parser.cpp b/tests/parser.cpp
index f06fefc..f3cb89f 100644
--- a/tests/parser.cpp
+++ b/tests/parser.cpp
@@ -508,6 +508,126 @@ TEST_CASE("not member list", "[parser, member]")
     REQUIRE(eval.value().deepEq(expected_eval));
 }
 
+TEST_CASE("member string found", "[parser, member]")
+{
+    auto input = R"("foo" in "foobar")"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foo")));
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foobar")));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::Member});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(true);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
+TEST_CASE("member string not found", "[parser, member]")
+{
+    auto input = R"("baz" in "foobar")"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("baz")));
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foobar")));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::Member});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(false);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
+TEST_CASE("not member string found", "[parser, member]")
+{
+    auto input = R"("foo" not in "foobar")"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foo")));
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foobar")));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::NotMember});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(false);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
+TEST_CASE("not member string not found", "[parser, member]")
+{
+    auto input = R"("baz" not in "foobar")"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("baz")));
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foobar")));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::NotMember});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(true);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
+TEST_CASE("member string in list", "[parser, member]")
+{
+    auto input = R"("foo" in ["foo", "bar"])"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("foo")));
+    expressions.push_back(make_list_expr(make_expr_ptr<StringExpr>(std::string("foo")), make_expr_ptr<StringExpr>(std::string("bar"))));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::Member});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(true);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
+TEST_CASE("not member string in list", "[parser, member]")
+{
+    auto input = R"("baz" not in ["foo", "bar"])"sv;
+
+    std::vector<ExprPtr> expressions;
+    expressions.push_back(make_expr_ptr<StringExpr>(std::string("baz")));
+    expressions.push_back(make_list_expr(make_expr_ptr<StringExpr>(std::string("foo")), make_expr_ptr<StringExpr>(std::string("bar"))));
+
+    const auto expected_ast = make_expr_ptr<ComparisonChainExpr>(std::move(expressions), std::vector{ComparisonOperators::NotMember});
+    const auto expected_eval = CuraFormulaeEngine::eval::Value(true);
+
+    const auto message = CuraFormulaeEngine::parser::parse(input);
+    REQUIRE(message.has_value());
+    const auto &ast = message.value();
+    REQUIRE(ast.deepEq(expected_ast));
+    const auto eval = ast.evaluate(&CuraFormulaeEngine::env::std_env);
+    REQUIRE(eval.has_value());
+    REQUIRE(eval.value().deepEq(expected_eval));
+}
+
 TEST_CASE("expression or operator", "[parser, expression]")
 {
     auto input = "True or False"sv;