diff --git a/src/lexer.cpp b/src/lexer.cpp
index cd62d75..5627689 100644
--- a/src/lexer.cpp
+++ b/src/lexer.cpp
@@ -48,11 +48,20 @@ int obelisk::Lexer::getToken()
         return kTokenIdentifier;
     }
 
-    if (isdigit(lastChar) || lastChar == '.')
+    if (isdigit(lastChar))
     {
+        bool firstPeriod = false;
         std::string numberStr;
         do
         {
+            if (firstPeriod && lastChar == '.')
+            {
+                throw obelisk::LexerException("invalid double value");
+            }
+            else if (!firstPeriod && lastChar == '.')
+            {
+                firstPeriod = true;
+            }
             numberStr += lastChar;
             lastChar = getchar();
         }
diff --git a/src/lexer.h b/src/lexer.h
index c82b949..9c4308d 100644
--- a/src/lexer.h
+++ b/src/lexer.h
@@ -3,7 +3,6 @@
 
 #include <string>
 
-// TODO: add error handling
 namespace obelisk
 {
     class Lexer
@@ -41,6 +40,28 @@ namespace obelisk
             const std::string& getIdentifier();
             double getNumberValue();
     };
+
+    class LexerException : public std::exception
+    {
+        private:
+            const std::string errorMessage_;
+
+        public:
+            LexerException() :
+                errorMessage_("an unknown error ocurred")
+            {
+            }
+
+            LexerException(const std::string& errorMessage) :
+                errorMessage_(errorMessage)
+            {
+            }
+
+            const char* what() const noexcept
+            {
+                return errorMessage_.c_str();
+            }
+    };
 } // namespace obelisk
 
 #endif