diff --git a/.gitignore b/.gitignore
index d3e9155..1a0b942 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,9 @@
-# ignore config.json
+# Ignore custom configuration file.
 config.json
 
+# Ignore test files.
+sample-out.png
+
 ### Python template
 # Byte-compiled / optimized / DLL files
 __pycache__/
diff --git a/brother_ql_web/web.py b/brother_ql_web/web.py
index 35fb374..ce20a1e 100644
--- a/brother_ql_web/web.py
+++ b/brother_ql_web/web.py
@@ -88,7 +88,12 @@ def get_label_parameters(
             font_family = ""
             font_style = ""
         else:
-            raise
+            raise ValueError(
+                "Could not find valid font specifier. Please pass the `font_family` "
+                "parameter with the family and style in the format `Roboto (Medium)`, "
+                "where Roboto is the family name and Medium the corresponding font "
+                "style."
+            )
     context = {
         "text": d.get("text", ""),
         "image": _save_to_bytes(request.files.get("image")),
@@ -141,7 +146,7 @@ def print_text() -> dict[str, bool | str]:
 
     try:
         parameters = get_label_parameters(bottle.request)
-    except (AttributeError, LookupError) as e:
+    except (AttributeError, LookupError, ValueError) as e:
         return_dict["error"] = str(e)
         return return_dict
 
@@ -169,7 +174,7 @@ def print_image() -> dict[str, bool | str]:
 
     try:
         parameters = get_label_parameters(bottle.request, should_be_file=True)
-    except (AttributeError, LookupError) as e:
+    except (AttributeError, LookupError, ValueError) as e:
         return_dict["error"] = str(e)
         return return_dict
 
diff --git a/tests/test_web.py b/tests/test_web.py
index 5986592..6bcdb29 100644
--- a/tests/test_web.py
+++ b/tests/test_web.py
@@ -245,7 +245,10 @@ def test_error(self) -> None:
         self.assertEqual(
             (
                 b'{"success": false, '
-                b"\"error\": \"'NoneType' object has no attribute 'rpartition'\"}"
+                b'"error": "Could not find valid font specifier. '
+                b'Please pass the `font_family` parameter with the family and style '
+                b'in the format `Roboto (Medium)`, where Roboto is the family name '
+                b'and Medium the corresponding font style."}'
             ),
             response.content,
         )