diff --git a/src/textual/css/_style_properties.py b/src/textual/css/_style_properties.py
index 6fba80edd..c8831e3a3 100644
--- a/src/textual/css/_style_properties.py
+++ b/src/textual/css/_style_properties.py
@@ -814,7 +814,7 @@ class FractionalProperty:
 
         if isinstance(value, float):
             float_value = value
-        elif isinstance(value, str):
+        elif isinstance(value, str) and value.endswith("%"):
             float_value = float(Scalar.parse(value).value) / 100
         else:
             raise StyleTypeError(
diff --git a/tests/css/test_styles.py b/tests/css/test_styles.py
index 967f27f18..bb4417894 100644
--- a/tests/css/test_styles.py
+++ b/tests/css/test_styles.py
@@ -2,6 +2,7 @@ import pytest
 from rich.color import Color
 from rich.style import Style
 
+from textual.css.errors import StyleTypeError
 from textual.css.styles import Styles, RenderStyles
 from textual.dom import DOMNode
 
@@ -147,3 +148,9 @@ def test_opacity_set_then_get(set_value, expected):
     styles = RenderStyles(DOMNode(), Styles(), Styles())
     styles.opacity = set_value
     assert styles.opacity == expected
+
+
+def test_opacity_set_invalid_type_error():
+    styles = RenderStyles(DOMNode(), Styles(), Styles())
+    with pytest.raises(StyleTypeError):
+        styles.opacity = "invalid value"