diff --git a/src/nodes/eval.rs b/src/nodes/eval.rs
index d813e7c..cd37cb3 100644
--- a/src/nodes/eval.rs
+++ b/src/nodes/eval.rs
@@ -86,9 +86,11 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             n => return Err(Error::InvalidOperand(n)),
         };
         end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.pow_from(lhs),
-            n => return Err(Error::InvalidOperand(n.clone())),
+        match nodes.get_mut(start + operator) {
+            None => return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + operator)),
+
+            Some(Node::Number(n)) => n.pow_from(lhs),
+            Some(n) => return Err(Error::InvalidOperand(n.clone())),
         };
 
         nodes.remove(start + operator - 1);
@@ -104,9 +106,11 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             n => return Err(Error::InvalidOperand(n)),
         };
         end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.div_from(lhs),
-            n => return Err(Error::InvalidOperand(n.clone())),
+        match nodes.get_mut(start + operator) {
+            None => return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + operator)),
+
+            Some(Node::Number(n)) => n.div_from(lhs),
+            Some(n) => return Err(Error::InvalidOperand(n.clone())),
         };
 
         nodes.remove(start + operator - 1);
@@ -122,9 +126,11 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             n => return Err(Error::InvalidOperand(n)),
         };
         end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.mul_from(lhs),
-            n => return Err(Error::InvalidOperand(n.clone())),
+        match nodes.get_mut(start + operator) {
+            None => return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + operator)),
+
+            Some(Node::Number(n)) => n.mul_from(lhs),
+            Some(n) => return Err(Error::InvalidOperand(n.clone())),
         };
 
         nodes.remove(start + operator - 1);
@@ -140,9 +146,11 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             n => return Err(Error::InvalidOperand(n)),
         };
         end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.sub_from(lhs),
-            n => return Err(Error::InvalidOperand(n.clone())),
+        match nodes.get_mut(start + operator) {
+            None => return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + operator)),
+
+            Some(Node::Number(n)) => n.sub_from(lhs),
+            Some(n) => return Err(Error::InvalidOperand(n.clone())),
         };
 
         nodes.remove(start + operator - 1);
@@ -158,9 +166,11 @@ fn collapse_toplevel(nodes: &mut Vec<Node>, mut end: usize) -> Result<(), Error>
             n => return Err(Error::InvalidOperand(n)),
         };
         end -= 1;
-        match &mut nodes[start..end][operator] {
-            Node::Number(n) => n.add_from(lhs),
-            n => return Err(Error::InvalidOperand(n.clone())),
+        match nodes.get_mut(start + operator) {
+            None => return Err(Error::OutOfBounds(Nodes(nodes.clone()), start + operator)),
+
+            Some(Node::Number(n)) => n.add_from(lhs),
+            Some(n) => return Err(Error::InvalidOperand(n.clone())),
         };
 
         nodes.remove(start + operator - 1);