diff --git a/src/lib.rs b/src/lib.rs
index 22abdf7..843a046 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,7 +7,9 @@ pub enum ReadError {
 
 pub enum Error {
     Read(ReadError),
-    BadParam(String),
+    BadPopSize,
+    BadMut,
+    BadCrossRatio,
     BadMove(Pos),
 }
 
@@ -71,7 +73,9 @@ impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Error::Read(e) => write!(f, "{e}"),
-            Error::BadParam(s) => write!(f, "{s}"),
+            Error::BadPopSize => write!(f, "population size cannot be 0"),
+            Error::BadMut => write!(f, "mutation length too large"),
+            Error::BadCrossRatio => write!(f, "crossover ratio cannot be 0"),
             Error::BadMove(pos) => write!(f, "invalid move ({pos})"),
         }
     }
@@ -511,15 +515,15 @@ pub fn run(config: Config, rle: Option<String>) -> Result<(), Error> {
         .unwrap();
 
     if config.pop_size == 0 {
-        return Err(Error::BadParam("population size cannot be 0".into()));
+        return Err(Error::BadPopSize);
     }
 
     if config.mut_max > config.dna_size {
-        return Err(Error::BadParam("mutation length too large".into()));
+        return Err(Error::BadMut);
     }
 
     if config.cross_ratio == 0 {
-        return Err(Error::BadParam("crossover ratio cannot be 0".into()));
+        return Err(Error::BadCrossRatio);
     }
 
     let map = if let Some(rle) = rle {