diff --git a/Makefile b/Makefile
index 16fc303..49a4c16 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,10 @@
 CC = gcc
 CFLAGS=-Wall -DDEBUG
-SRC=src
-OBJ=obj
-SRCS=$(wildcard $(SRC)/*.c)
+SRCDIR=src
+OBJDIR=obj
+SRCS=$(wildcard $(SRCDIR)/*.c)
 # Substitute all .c with .o from SRCS
-OBJS=$(patsubst $(SRC)/%.c, $(OBJ)/%.o, $(SRCS))
+OBJS=$(patsubst $(SRCDIR)/%.c, $(OBJDIR)/%.o, $(SRCS))
 
 BINDIR=bin
 BIN=$(BINDIR)/clox
@@ -18,20 +18,20 @@ release: $(BIN)
 $(BIN): $(OBJS) $(BINDIR)
 	$(CC) $(CFLAGS) $(OBJS) -o $@
 
-$(OBJ)/%.o: $(SRC)/%.c $(OBJ)
+$(OBJDIR)/%.o: $(SRCDIR)/%.c $(OBJDIR)
 	$(CC) $(CFLAGS) -c $< -o $@
 
 $(BINDIR):
 	mkdir $@
 
-$(OBJ):
+$(OBJDIR):
 	mkdir $@
 
 .PHONY: clean
 .PHONY: test
 
 clean:
-	rm -r $(BINDIR)/* $(OBJ)/*
+	rm -r $(BINDIR)/* $(OBJDIR)/*
 
 test:
 	$(BIN) test.lox
\ No newline at end of file
diff --git a/src/interpreter.c b/src/interpreter.c
index 1091732..19f83e8 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -14,6 +14,7 @@ Object* VisitBinaryExpression(Expr*);
 int IsTruthy(Object*);
 int IsEqual(Object*, Object*);
 int ConcatStringObject(Object*, const Object*);
+void CheckNumberOperand(TokenType, int, ...);
 void PrintObject(Object*);
 
 void PrintObject(Object* o) {
@@ -53,7 +54,6 @@ void Interpret(Expr* exp) {
 }
 
 Object* VisitLiteralExpression(Expr* expr) {
-    printf("Literal: %p\n", (double *) expr->expression.Literal->literal);
     return CreateObject(expr->expression.Literal->literal, expr->expression.Literal->type);
 }
 
@@ -66,8 +66,6 @@ Object* VisitGroupingExpression(Expr* expression) {
 Object* VisitBinaryExpression(Expr* expression) {
     Object* left = Evaluate(expression->expression.Binary.left);
     Object* right = Evaluate(expression->expression.Binary.right);
-    PrintObject(left);
-    PrintObject(right);
     double computed_value;
 
     switch(expression->expression.Binary.op->type) {
@@ -108,6 +106,7 @@ Object* VisitBinaryExpression(Expr* expression) {
         return CreateObject(&computed_value, TRUE);
         //return IsEqual(left, right);
         case Minus:
+
             left->value.number -= right->value.number;
             FreeObject(right);
             return left;
@@ -159,6 +158,7 @@ Object* VisitUnaryExpression(Expr* expression) {
 
     switch (expression->expression.Unary.op->type) {
         case Minus:
+        CheckNumberOperand(expression->expression.Unary.op->type, right);//This needs to "throw" if the function fails.
         computed_value = -right->value.number;
         FreeObject(right);
         c = CreateObject(&computed_value, Number);
@@ -282,4 +282,19 @@ int IsEqual(Object* a, Object* b) {
     }
     
     return 0;
+}
+
+void CheckNumberOperand(TokenType operator, int operandCount, ...) {
+    va_list list;
+    va_start(list, operandCount);
+
+    for(int i = 0; i < operandCount; i++) {
+        Object* operand = va_arg(list, Object*);
+        if (operand->instance != INS_DOUBLE) {
+            va_end(list);
+            return; //TODO: "throw" runtime error "Operand must be a number."
+        }
+    }
+
+    va_end(list);
 }
\ No newline at end of file
diff --git a/src/parser.c b/src/parser.c
index 2469279..e8ffa69 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -157,11 +157,14 @@ int Match(int count, ...) {
 
     for(int i = 0; i < count; i++) {
         if(Check(va_arg(list, TokenType))) {
+            va_end(list);
             AdvanceParser();
             return 1;
         }
     }
 
+    va_end(list);
+
     return 0;
 }