diff --git a/src/nodes/eval.rs b/src/nodes/eval.rs
index ea06c2b..d813e7c 100644
--- a/src/nodes/eval.rs
+++ b/src/nodes/eval.rs
@@ -7,6 +7,8 @@ use crate::nodes::{Function, Node, Nodes, Operator};
 
 #[derive(Clone, Debug, thiserror::Error)]
 pub enum Error {
+    #[error("out of bounds: index {0}, {1}")]
+    OutOfBounds(Nodes, usize),
     #[error("invalid operand: {0}")]
     InvalidOperand(Node),
     #[error("missing argument, got: {0}")]
@@ -48,22 +50,27 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
         })
         .next()
     {
-        match &mut nodes[start..end][index + 1] {
-            Node::Number(n) => match variant {
-                Function::Sqrt => n.sqrt_mut(),
+        let Some(node) = nodes.get_mut(start + index + 1) else {
+            return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + index + 1));
+        };
 
-                Function::Ln => n.ln_mut(),
+        let Node::Number(n) = node else {
+            return Err(Error::MissingArgument(node.clone()));
+        };
 
-                Function::Sine => n.sin_mut(),
-                Function::ArcSine => n.asin_mut(),
-                Function::Cosine => n.cos_mut(),
-                Function::ArcCosine => n.acos_mut(),
-                Function::Tangent => n.tan_mut(),
-                Function::ArcTangent => n.atan_mut(),
-                Function::Secant => n.sec_mut(),
-                Function::CoSecant => n.csc_mut(),
-            },
-            n => return Err(Error::MissingArgument(n.clone())),
+        match variant {
+            Function::Sqrt => n.sqrt_mut(),
+
+            Function::Ln => n.ln_mut(),
+
+            Function::Sine => n.sin_mut(),
+            Function::ArcSine => n.asin_mut(),
+            Function::Cosine => n.cos_mut(),
+            Function::ArcCosine => n.acos_mut(),
+            Function::Tangent => n.tan_mut(),
+            Function::ArcTangent => n.atan_mut(),
+            Function::Secant => n.sec_mut(),
+            Function::CoSecant => n.csc_mut(),
         };
 
         nodes.remove(start + index);