diff --git a/compiler/parser.py b/compiler/parser.py
index ff967b7..43c9bdd 100644
--- a/compiler/parser.py
+++ b/compiler/parser.py
@@ -91,19 +91,21 @@ class Parser:
             raise UnexpectedTokenError(self.token, "variable identifier")
 
     @tracer.trace_method
-    def binary_op(self, operand_func: Callable[[], Value], operators: Dict[Tokens, Value]):
-        operand = operand_func()
+    def binary_op(self, operand_func: Callable[[bool], Value], operators: Dict[Tokens, Value], mandatory: bool = True):
+        operand = operand_func(mandatory)
+        if not operand and mandatory:
+            raise UnexpectedTokenError(operand, "operand")
 
         while operator := self.accept(*list(operators.keys())):
             node_type = operators[operator.kind]
-            operand2 = operand_func()
+            operand2 = operand_func(True)
             operand = node_type(operand, operand2)
             logger.debug(f"{node_type.__name__} of the following operands: {operand} and {operand2}")
 
         return operand
 
     @tracer.trace_method
-    def factor(self) -> Value:
+    def factor(self, mandatory: bool = False) -> Value:
         if self.accept(Tokens.Parens_Left):
             v = self.expression()
             self.expect(Tokens.Parens_Right)
@@ -112,42 +114,44 @@ class Parser:
             return num
         elif variable := self.variable():
             return variable
-        else:
+        elif mandatory:
             raise UnexpectedTokenError(self.token, "parenthesized expression, number or variable")
 
     @tracer.trace_method
-    def term(self) -> Value:
+    def term(self, mandatory: bool = False) -> Value:
         return self.binary_op(self.factor, operators={
             Tokens.Op_Multiply: Product,
             Tokens.Op_Divide: Division,
-        })
+        }, mandatory=mandatory)
 
     @tracer.trace_method
-    def summation(self) -> Sum:
+    def summation(self, mandatory: bool = True) -> Sum:
         return self.binary_op(self.term, operators={
             Tokens.Op_Plus: Sum,
             Tokens.Op_Minus: Sub,
-        })
+        }, mandatory=mandatory)
 
     @tracer.trace_method
     def assignment(self, mandatory: bool = False) -> Assignment:
         if ident := self.identifier(mandatory):
             self.expect(Tokens.Equal)
-            expr = self.expression()
+            expr = self.expression(mandatory=True)
             return Assignment(ident, expr)
         elif mandatory:
             raise UnexpectedTokenError(self.token, "assignment")
 
     @tracer.trace_method
-    def expression(self) -> Value:
-        if self.peek_several(Tokens.Identifier, Tokens.Equal):
-            return Expression(self.assignment())
-        else:
-            return Expression(self.summation())
+    def expression(self, mandatory: bool = False) -> Value:
+        if self.peek_several(Tokens.Identifier, Tokens.Equal) and (assignment := self.assignment(mandatory)):
+            return Expression(assignment)
+        elif summation := self.summation(mandatory):
+            return Expression(summation)
+        elif mandatory:
+            raise UnexpectedTokenError(self.token, "expression")
 
     @tracer.trace_method
     def root(self) -> Node:
-        return self.expression()
+        return self.expression(mandatory=True)
 
     def parse(self) -> Node:
         try: