diff --git a/fj-kernel/src/algorithms/approximation.rs b/fj-kernel/src/algorithms/approximation.rs
index 624f47d9a..29a467bf3 100644
--- a/fj-kernel/src/algorithms/approximation.rs
+++ b/fj-kernel/src/algorithms/approximation.rs
@@ -184,11 +184,13 @@ impl Tolerance {
     /// Construct a `Tolerance` from a [`Scalar`]
     ///
     /// Returns an error, if the passed scalar is not larger than zero.
-    pub fn from_scalar(scalar: impl Into<Scalar>) -> Result<Self, Scalar> {
+    pub fn from_scalar(
+        scalar: impl Into<Scalar>,
+    ) -> Result<Self, InvalidTolerance> {
         let scalar = scalar.into();
 
         if scalar <= Scalar::ZERO {
-            return Err(scalar);
+            return Err(InvalidTolerance(scalar));
         }
 
         Ok(Self(scalar))
@@ -209,6 +211,10 @@ where
     }
 }
 
+#[derive(Debug, thiserror::Error)]
+#[error("Invalid tolerance ({0}); must be above zero")]
+pub struct InvalidTolerance(Scalar);
+
 #[cfg(test)]
 mod tests {
     use fj_math::{Point, Scalar};