diff --git a/src/manifold/mod.rs b/src/manifold/mod.rs
index 6e5c6fa4..2e4c6c8e 100644
--- a/src/manifold/mod.rs
+++ b/src/manifold/mod.rs
@@ -41,15 +41,18 @@ impl<S: Data> Manifold<S> {
         if !pos.len().is_multiple_of(3) { return Err("pos must be a multiple of 3".into()); }
         if !idx.len().is_multiple_of(3) { return Err("idx must be a multiple of 3".into()); }
         if !val.is_empty() && val.len() != idx.len() { return Err("val must hold one value per corner".into()); }
+        let nv = pos.len() / 3;
+        if idx.iter().any(|&i| i >= nv) { return Err("idx must refer to vertices in pos".into()); }
 
         // dedup vertices
-        let mut hash = HashMap::with_capacity(pos.len() / 3);
-        let mut weld = Vec::with_capacity(pos.len() / 3);
-        let mut rmap = vec![0; pos.len()];
+        let mut hash = HashMap::with_capacity(nv);
+        let mut weld = Vec::with_capacity(nv);
+        let mut rmap = vec![0; nv];
         let mut hs   = Vec::with_capacity(idx.len());
 
         for (i, p) in pos.chunks(3).enumerate() {
             let v = Vec3::new(p[0] as Real, p[1] as Real, p[2] as Real);
+            if !v.is_finite() { return Err("pos must not contain NaN or infinite coordinates".into()); }
             let k = (v.x.to_bits(), v.y.to_bits(), v.z.to_bits());
             if let Some(&w) = hash.get(&k) { rmap[i] = w; } else {
                 let n = weld.len();
diff --git a/src/tests.rs b/src/tests.rs
index 7ec812a1..31cee227 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -763,3 +763,41 @@ mod test_values {
     }
 
 }
+
+#[cfg(test)]
+mod test_input_validation {
+    use crate::prelude::Manifold;
+
+    // A unit tetrahedron: valid input, so each test changes one thing.
+    const POS: [f64; 12] = [0., 0., 0., 1., 0., 0., 0., 1., 0., 0., 0., 1.];
+    const IDX: [usize; 12] = [0, 2, 1, 0, 1, 3, 0, 3, 2, 1, 2, 3];
+
+    #[test]
+    fn test_valid_tet_is_accepted() {
+        assert!(Manifold::new(&POS, &IDX).is_ok());
+    }
+
+    #[test]
+    fn test_index_past_end_of_pos_is_an_error() {
+        let mut idx = IDX;
+        idx[0] = 99;
+        assert!(Manifold::new(&POS, &idx).is_err());
+    }
+
+    #[test]
+    fn test_index_past_last_vertex_is_an_error() {
+        // 4 vertices, but `pos.len()` is 12: index 5 used to be read as vertex 0.
+        let mut idx = IDX;
+        idx[0] = 5;
+        assert!(Manifold::new(&POS, &idx).is_err());
+    }
+
+    #[test]
+    fn test_non_finite_coordinates_are_errors() {
+        for bad in [f64::NAN, f64::INFINITY, f64::NEG_INFINITY] {
+            let mut pos = POS;
+            pos[3] = bad;
+            assert!(Manifold::new(&pos, &IDX).is_err(), "{bad} was accepted");
+        }
+    }
+}