diff --git a/src/arx/lexer.py b/src/arx/lexer.py
index 4c0f8d2..6ad9a46 100644
--- a/src/arx/lexer.py
+++ b/src/arx/lexer.py
@@ -204,6 +204,16 @@ def get_next_token(self) -> Token:
         return self.cur_tok
 
 
+class LexerError(Exception):
+    """Custom exception for lexer errors."""
+
+    def __init__(self, message: str, location: SourceLocation):
+        super().__init__(
+            f"{message} at line {location.line}, col {location.col}"
+        )
+        self.location = location
+
+
 class Lexer:
     """
     Lexer class for tokenizing known variables.
@@ -311,7 +321,16 @@ def get_token(self) -> Token:
         # Number: [0-9.]+
         if self.last_char.isdigit() or self.last_char == ".":
             num_str = ""
+            dot_count = 0
+
             while self.last_char.isdigit() or self.last_char == ".":
+                if self.last_char == ".":
+                    dot_count += 1
+                    if dot_count > 1:
+                        raise LexerError(
+                            "Invalid number format: multiple decimal points",
+                            self.lex_loc,
+                        )
                 num_str += self.last_char
                 self.last_char = self.advance()