diff --git a/src/class.rs b/src/class.rs
index 5698baf..4edaddb 100644
--- a/src/class.rs
+++ b/src/class.rs
@@ -50,8 +50,7 @@ impl<'a> Class<'a> {
             .replace('%', "\\%")
     }
 
-    /// TODO return result
-    pub(crate) fn generate(&self, z: &Zephyr) -> String {
+    pub(crate) fn generate(&self, z: &Zephyr) -> Result<String, &'static str> {
         let name = z
             .names
             .get(self.name)
@@ -61,11 +60,11 @@ impl<'a> Class<'a> {
 
         if let Some(val) = self.value {
             let val = z.values.get(val).map(AsRef::as_ref).unwrap_or(val);
-            format!("{selector} {{ {name}: {val}; }}",)
+            Ok(format!("{selector} {{ {name}: {val}; }}",))
         } else if let Some(v) = z.rules.get(name) {
-            format!("{selector} {{ {v} }}",)
+            Ok(format!("{selector} {{ {v} }}",))
         } else {
-            panic!("{name} is not a no-variable rule, and no variables were provided");
+            Err("{name} is not a no-variable rule, and no variables were provided")
         }
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 0b0d6d1..f37c70a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -52,13 +52,16 @@ impl Zephyr {
             .into_iter()
             .flat_map(|s| s.split_ascii_whitespace())
             // TODO skip duplicates, use hashset or smth
-            .flat_map(|c| self.generate_class(c))
+            // we ignore errors
+            .flat_map(|c| self.generate_class(c).ok().flatten())
             .collect::<Vec<_>>()
             .join("")
     }
 
-    pub fn generate_class(&self, class: &str) -> Option<String> {
-        parse_class(class).map(|c| c.generate(self))
+    /// this one returns an error if parsing or generating fails
+    // TODO add an error type
+    pub fn generate_class(&self, class: &str) -> Result<Option<String>, &'static str> {
+        parse_class(class).map(|c| c.generate(self)).transpose()
     }
 }