diff --git a/ast/src/ast.rs b/ast/src/ast.rs
index 8506abf29d..a41bf13806 100644
--- a/ast/src/ast.rs
+++ b/ast/src/ast.rs
@@ -159,12 +159,11 @@ impl<'ast> FromPest<'ast> for Expression<'ast> {
     type Rule = Rule;
 
     fn from_pest(pest: &mut Pairs<'ast, Rule>) -> Result<Self, ConversionError<Void>> {
-        let mut clone = pest.clone();
-        let pair = clone.next().ok_or(::from_pest::ConversionError::NoMatch)?;
+        let pair = pest.peek().ok_or(::from_pest::ConversionError::NoMatch)?;
         match pair.as_rule() {
             Rule::expression => {
-                // Transfer iterated state to pest.
-                *pest = clone;
+                // advance the iterator
+                pest.next();
                 Ok(PRECEDENCE_CLIMBER.climb(pair.into_inner(), parse_term, binary_expression))
             }
             _ => Err(ConversionError::NoMatch),