diff --git a/2020/18.cpp b/2020/18.cpp
index d35564e..804e06a 100644
--- a/2020/18.cpp
+++ b/2020/18.cpp
@@ -23,30 +23,73 @@ auto Eval(
   std::vector<std::int64_t> vals;
   
   auto const eval1 = [&](){
-    char o = ops.back(); ops.pop_back();
-    std::int64_t y = vals.back(); vals.pop_back();
-    std::int64_t x = vals.back(); vals.pop_back();
-    vals.push_back(cfg[o].second(x,y));
+    if (ops.size() < 1 || vals.size() < 2) {
+      throw std::runtime_error{"parse error"};
+    }
+    if (auto opit = cfg.find(ops.back()); opit != cfg.end()) {
+      std::int64_t y = vals.back(); vals.pop_back();
+      std::int64_t x = vals.back(); vals.pop_back();
+      vals.push_back(opit->second.second(x,y));
+      ops.pop_back();
+    } else {
+      throw std::runtime_error{"parse error"};
+    }
   };
 
-  for (auto it = input.begin(); it != input.end(); it++) {
-    char const c = *it;
+  bool expect_operator = false;
+
+  auto it = input.begin();
+  while (it != input.end()) {
+    char const c = *it++;
+
+    // skip whitespace
     if (c == ' ') continue;
+
+    // number literal
     if (std::isdigit(c)) {
-      vals.push_back(c - '0');
-    } else if ('(' == c) {
+      if (expect_operator) { throw std::runtime_error{"unexpected literal"}; }
+      expect_operator = true;
+
+      std::int64_t acc = c - '0';
+      while (it != input.end() && std::isdigit(*it)) {
+        acc = acc*10 + (*it++ - '0');
+      }
+      vals.push_back(acc);
+      continue;
+    }
+    
+    // begin subexpression
+    if ('(' == c) {
+      if (expect_operator) { throw std::runtime_error{"unexpected '('"}; }
       ops.push_back('(');
-    } else if (')' == c) {
-      while (ops.back() != '(') eval1();
-      ops.pop_back();
-    } else if (auto opit = cfg.find(c); opit != cfg.end()) {
+      continue;
+    }
+    
+    // end subexpression
+    if (')' == c) {
+      if (!expect_operator) { throw std::runtime_error{"unexpected ')'"}; }
+      while (!ops.empty() && ops.back() != '(') eval1();
+      if (ops.empty()) throw std::runtime_error{"parse error"};
+      ops.pop_back(); // '('
+      continue;
+    }
+    
+    // binary operator
+    if (auto opit = cfg.find(c); opit != cfg.end()) {
+      if (!expect_operator) { throw std::runtime_error{"unexpected operator"}; }
+      expect_operator = false;
       while (!ops.empty() && cfg[ops.back()].first >= opit->second.first) eval1();
       ops.push_back(c);
+      continue;
     }
+
+    throw std::runtime_error{"unknown character"};
   }
 
   while (!ops.empty()) eval1();
 
+  if (vals.empty()) { throw std::runtime_error{"empty input"}; }
+
   return vals.back();
 }
 
@@ -63,21 +106,36 @@ auto Part2(std::string const& line) {
 TEST_SUITE("documented examples") {
 
 TEST_CASE("part 1") {
-  REQUIRE(Part1("1 + 2 * 3 + 4 * 5 + 6") == 71);
-  REQUIRE(Part1("1 + (2 * 3) + (4 * (5 + 6))") == 51);
-  REQUIRE(Part1("2 * 3 + (4 * 5)") == 26);
-  REQUIRE(Part1("5 + (8 * 3 + 9 + 3 * 4 * 3)") == 437);
-  REQUIRE(Part1("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))") == 12240);
-  REQUIRE(Part1("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2") == 13632);
+  CHECK(Part1("1 + 2 * 3 + 4 * 5 + 6") == 71);
+  CHECK(Part1("1 + (2 * 3) + (4 * (5 + 6))") == 51);
+  CHECK(Part1("2 * 3 + (4 * 5)") == 26);
+  CHECK(Part1("5 + (8 * 3 + 9 + 3 * 4 * 3)") == 437);
+  CHECK(Part1("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))") == 12240);
+  CHECK(Part1("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2") == 13632);
 }
 
 TEST_CASE("part 2") {
-  REQUIRE(Part2("1 + 2 * 3 + 4 * 5 + 6") == 231);
-  REQUIRE(Part2("1 + (2 * 3) + (4 * (5 + 6))") == 51);
-  REQUIRE(Part2("2 * 3 + (4 * 5)") == 46);
-  REQUIRE(Part2("5 + (8 * 3 + 9 + 3 * 4 * 3)") == 1445);
-  REQUIRE(Part2("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))") == 669060);
-  REQUIRE(Part2("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2") == 23340);
+  CHECK(Part2("1 + 2 * 3 + 4 * 5 + 6") == 231);
+  CHECK(Part2("1 + (2 * 3) + (4 * (5 + 6))") == 51);
+  CHECK(Part2("2 * 3 + (4 * 5)") == 46);
+  CHECK(Part2("5 + (8 * 3 + 9 + 3 * 4 * 3)") == 1445);
+  CHECK(Part2("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))") == 669060);
+  CHECK(Part2("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2") == 23340);
+}
+
+TEST_CASE("errors") {
+  CHECK_THROWS_AS(Part1(""), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 + ()"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("(1"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 (2)"), std::runtime_error);
+  CHECK_THROWS_AS(Part1(")"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1)"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 2"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 +"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("+ 1"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 /"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 / 2"), std::runtime_error);
+  CHECK_THROWS_AS(Part1("1 + * 2"), std::runtime_error);
 }
 
 }