diff --git a/third_party/a8/a8/lisp.cc b/third_party/a8/a8/lisp.cc
index d86e3d4..842ab87 100644
--- a/third_party/a8/a8/lisp.cc
+++ b/third_party/a8/a8/lisp.cc
@@ -150,7 +150,7 @@ namespace a8
                      if (params->size() > 0) {
                          result = std::any_cast<Atom>(params->at(0)->value).val;
                          if (params->size() > 1) {
-                             for (int i = 1; i < params->size(); ++i) {
+                             for (size_t i = 1; i < params->size(); ++i) {
                                  auto& param = params->at(i);
                                  Atom&& atom = std::any_cast<Atom>(param->value);
                                  result /= atom.val;
@@ -229,13 +229,13 @@ namespace a8
          */
         int Expr::GetToken(const std::string& script, int& pos, std::string& token)
         {
-            if (pos >= script.size()) {
+            if ((size_t)pos >= script.size()) {
                 return -1;
             }
-            while (pos < script.size() && isspace(script[pos])) {
+            while ((size_t)pos < script.size() && isspace(script.at((size_t)pos))) {
                 ++pos;
             }
-            if (pos >= script.size()) {
+            if ((size_t)pos >= script.size()) {
                 return -1;
             }
             if (script[pos] == '(') {
@@ -248,10 +248,10 @@ namespace a8
                 token = ")";
                 return 1;
             }
-            while (pos < script.size() &&
+            while ((size_t)pos < script.size() &&
                    !isspace(script[pos]) &&
-                   script[pos] != ')' &&
-                   script[pos] != '(') {
+                   script[(size_t)pos] != ')' &&
+                   script[(size_t)pos] != '(') {
                 token += script[pos];
                 ++pos;
             }