diff --git a/parser/parser.yy b/parser/parser.yy
index c658dce..e950390 100644
--- a/parser/parser.yy
+++ b/parser/parser.yy
@@ -161,7 +161,7 @@ extrude: EXTRUDE LCURLY general_items RCURLY {
             }
        ;
 
-general_items: /* empty */
+general_items: /* empty */ { $$ = NULL; }
              | general_item general_items {
                 $$ = new ItemsNode();
                 $$->addChild($1);
@@ -309,7 +309,7 @@ shape_ref: SHAPE IDENTIFIER shape_ref_more {
          }
          ;
 
-shape_ref_more: /* empty */
+shape_ref_more: /* empty */ { $$ = NULL; }
               | LCURLY general_items RCURLY { $$ = $2; }
               ;
 
@@ -375,8 +375,8 @@ expression: expression TIMES expression { $$ = new BinOpNode('*', $1, $3); }
           | LPAREN expression RPAREN    { $$ = $2; }
           ;
 
-maybe_expression: /* empty */
-                | expression        { $$ = $1; }
+maybe_expression: /* empty */           { $$ = NULL; }
+                | expression            { $$ = $1; }
                 ;
 
 stmt_expression: assignment              { $$ = $1; }
@@ -449,7 +449,7 @@ if: IF LPAREN bool_expression RPAREN LCURLY general_items RCURLY if_more {
   }
   ;
 
-if_more: /* empty */
+if_more: /* empty */ { $$ = NULL; }
        | ELSIF LPAREN bool_expression RPAREN LCURLY general_items RCURLY if_more {
            $$ = new IfNode($3, $8);
            $$->addChildren($6);
@@ -465,7 +465,7 @@ function_call: IDENTIFIER LPAREN function_call_parameters RPAREN {
              }
              ;
 
-function_call_parameters: /* empty */
+function_call_parameters: /* empty */ { $$ = NULL; }
                         | expression function_call_more_parameters {
                             $$ = new ItemsNode();
                             $$->addChild($1);
@@ -473,7 +473,7 @@ function_call_parameters: /* empty */
                         }
                         ;
 
-function_call_more_parameters: /* empty */
+function_call_more_parameters: /* empty */ { $$ = NULL; }
                              | COMMA expression function_call_more_parameters {
                                 $$ = new ItemsNode();
                                 $$->addChild($2);