diff --git a/calculator/ui/gui.py b/calculator/ui/gui.py
index 221ca9b..79e1c1d 100644
--- a/calculator/ui/gui.py
+++ b/calculator/ui/gui.py
@@ -83,8 +83,10 @@ def on_button_press(self, button):
                 self._calc.open_parenthesis()
             case "cos":
                 self._calc.cos()
+                self._calc.open_parenthesis()
             case "sin":
                 self._calc.sin()
+                self._calc.open_parenthesis()
             case _:
                 self._calc.digit(button.text)
         self.display.text = self._calc.expression or "0"
diff --git a/tests/test_gui.py b/tests/test_gui.py
index 9192589..ae3d74a 100644
--- a/tests/test_gui.py
+++ b/tests/test_gui.py
@@ -78,7 +78,7 @@ def test_expression_with_cos(self):
         self.press_button(")")
         self.assert_display("cos(0)")
         self.press_button("=")
-        self.assert_display("1")
+        self.assert_display("1.0")
     
     def test_expression_with_sin(self):
         # sin(0) = 0
@@ -87,4 +87,4 @@ def test_expression_with_sin(self):
         self.press_button(")")
         self.assert_display("sin(0)")
         self.press_button("=")
-        self.assert_display("0")
+        self.assert_display("0.0")
diff --git a/tests/test_model.py b/tests/test_model.py
index 1acf17e..790984c 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -94,6 +94,7 @@ def test_cosine(self):
         # cos(0) = 1
         self.assertEqual("", self.calculator.expression)
         self.calculator.cos()
+        self.calculator.open_parenthesis()
         self.assertEqual("cos(", self.calculator.expression)
         self.calculator.digit(0)
         self.calculator.close_parenthesis()
@@ -104,6 +105,7 @@ def test_sine(self):
         # sin(0) = 0
         self.assertEqual("", self.calculator.expression)
         self.calculator.sin()
+        self.calculator.open_parenthesis()
         self.assertEqual("sin(", self.calculator.expression)
         self.calculator.digit(0)
         self.calculator.close_parenthesis()