diff --git a/include/nlohmann/detail/input/parser.hpp b/include/nlohmann/detail/input/parser.hpp
index 1cd6868f..67bdcd49 100644
--- a/include/nlohmann/detail/input/parser.hpp
+++ b/include/nlohmann/detail/input/parser.hpp
@@ -96,7 +96,9 @@ class parser
 
         /// a parse error occurred
         /// the byte position and the last token are reported
-        virtual bool parse_error(int position, const std::string& last_token) = 0;
+        virtual bool parse_error(std::size_t position, const std::string& last_token) = 0;
+
+        virtual ~SAX() = default;
     };
 
     using parser_callback_t =
@@ -585,7 +587,7 @@ class parser
         {
             case token_type::begin_object:
             {
-                if (not sax->start_object(-1))
+                if (not sax->start_object(std::size_t(-1)))
                 {
                     return false;
                 }
@@ -654,7 +656,7 @@ class parser
 
             case token_type::begin_array:
             {
-                if (not sax->start_array(-1))
+                if (not sax->start_array(std::size_t(-1)))
                 {
                     return false;
                 }
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 79581142..9c04cd9b 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -3219,7 +3219,9 @@ class parser
 
         /// a parse error occurred
         /// the byte position and the last token are reported
-        virtual bool parse_error(int position, const std::string& last_token) = 0;
+        virtual bool parse_error(std::size_t position, const std::string& last_token) = 0;
+
+        virtual ~SAX() = default;
     };
 
     using parser_callback_t =
@@ -3708,7 +3710,7 @@ class parser
         {
             case token_type::begin_object:
             {
-                if (not sax->start_object(-1))
+                if (not sax->start_object(std::size_t(-1)))
                 {
                     return false;
                 }
@@ -3777,7 +3779,7 @@ class parser
 
             case token_type::begin_array:
             {
-                if (not sax->start_array(-1))
+                if (not sax->start_array(std::size_t(-1)))
                 {
                     return false;
                 }
diff --git a/test/src/unit-class_parser.cpp b/test/src/unit-class_parser.cpp
index 27c1ee8d..94acaaa5 100644
--- a/test/src/unit-class_parser.cpp
+++ b/test/src/unit-class_parser.cpp
@@ -61,7 +61,7 @@ class SaxEventLogger : public nlohmann::json::SAX
         return true;
     }
 
-    bool number_float(json::number_float_t val, const std::string& s) override
+    bool number_float(json::number_float_t, const std::string& s) override
     {
         events.push_back("number_float(" + s + ")");
         return true;
@@ -75,7 +75,7 @@ class SaxEventLogger : public nlohmann::json::SAX
 
     bool start_object(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_object()");
         }
@@ -100,7 +100,7 @@ class SaxEventLogger : public nlohmann::json::SAX
 
     bool start_array(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_array()");
         }
@@ -117,13 +117,13 @@ class SaxEventLogger : public nlohmann::json::SAX
         return true;
     }
 
-    bool binary(const std::vector<uint8_t>& vec) override
+    bool binary(const std::vector<uint8_t>&) override
     {
         events.push_back("binary()");
         return true;
     }
 
-    bool parse_error(int position, const std::string& last_token) override
+    bool parse_error(std::size_t position, const std::string&) override
     {
         errored = true;
         events.push_back("parse_error(" + std::to_string(position) + ")");
diff --git a/test/src/unit-deserialization.cpp b/test/src/unit-deserialization.cpp
index 9f0bc175..603773b9 100644
--- a/test/src/unit-deserialization.cpp
+++ b/test/src/unit-deserialization.cpp
@@ -60,7 +60,7 @@ struct SaxEventLogger : public nlohmann::json::SAX
         return true;
     }
 
-    bool number_float(json::number_float_t val, const std::string& s) override
+    bool number_float(json::number_float_t, const std::string& s) override
     {
         events.push_back("number_float(" + s + ")");
         return true;
@@ -74,7 +74,7 @@ struct SaxEventLogger : public nlohmann::json::SAX
 
     bool start_object(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_object()");
         }
@@ -99,7 +99,7 @@ struct SaxEventLogger : public nlohmann::json::SAX
 
     bool start_array(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_array()");
         }
@@ -116,13 +116,13 @@ struct SaxEventLogger : public nlohmann::json::SAX
         return true;
     }
 
-    bool binary(const std::vector<uint8_t>& vec) override
+    bool binary(const std::vector<uint8_t>&) override
     {
         events.push_back("binary()");
         return true;
     }
 
-    bool parse_error(int position, const std::string& last_token) override
+    bool parse_error(std::size_t position, const std::string&) override
     {
         events.push_back("parse_error(" + std::to_string(position) + ")");
         return false;
@@ -135,7 +135,7 @@ struct SaxEventLoggerExitAfterStartObject : public SaxEventLogger
 {
     bool start_object(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_object()");
         }
@@ -160,7 +160,7 @@ struct SaxEventLoggerExitAfterStartArray : public SaxEventLogger
 {
     bool start_array(std::size_t elements) override
     {
-        if (elements == -1)
+        if (elements == std::size_t(-1))
         {
             events.push_back("start_array()");
         }