diff --git a/src/backends/mock_main.rs b/src/backends/mock_main.rs
index 371871e..cc15edd 100644
--- a/src/backends/mock_main.rs
+++ b/src/backends/mock_main.rs
@@ -8,6 +8,7 @@ use itertools::Itertools;
 use plonky2::hash::poseidon::PoseidonHash;
 use plonky2::plonk::config::Hasher;
 use std::any::Any;
+use std::error::Error;
 use std::fmt;
 
 pub struct MockProver {}
@@ -30,6 +31,23 @@ impl OperationArg {
     }
 }
 
+#[derive(Clone, Debug, PartialEq, Eq)]
+enum OperationArgError {
+    KeyNotFound,
+    StatementNotFound,
+}
+
+impl std::fmt::Display for OperationArgError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            OperationArgError::KeyNotFound => write!(f, "Key not found"),
+            OperationArgError::StatementNotFound => write!(f, "Statement not found"),
+        }
+    }
+}
+
+impl std::error::Error for OperationArgError {}
+
 #[derive(Clone, Debug, PartialEq, Eq)]
 struct Operation(pub NativeOperation, pub Vec<OperationArg>);
 
@@ -236,11 +254,13 @@ impl MockMainPod {
         statements
     }
 
-    fn find_op_arg(statements: &[Statement], op_arg: &middleware::OperationArg) -> OperationArg {
+    pub fn find_op_arg(
+        statements: &[Statement],
+        op_arg: &middleware::OperationArg,
+    ) -> Result<OperationArg, OperationArgError> {
         match op_arg {
-            middleware::OperationArg::None => OperationArg::None,
-            middleware::OperationArg::Key(k) => OperationArg::Index(
-                // TODO: Error handling when the key is not found in any ValueOf statement
+            middleware::OperationArg::None => Ok(OperationArg::None),
+            middleware::OperationArg::Key(k) => {
                 statements
                     .iter()
                     .enumerate()
@@ -251,16 +271,17 @@ impl MockMainPod {
                         },
                         _ => None,
                     })
-                    .unwrap(),
-            ),
-            middleware::OperationArg::Statement(st) => OperationArg::Index(
-                // TODO: Error handling when the statement is not found
+                    .map(OperationArg::Index)
+                    .ok_or(OperationArgError::KeyNotFound)
+            }
+            middleware::OperationArg::Statement(st) => {
                 statements
                     .iter()
                     .enumerate()
                     .find_map(|(i, s)| (s == st).then_some(i))
-                    .unwrap(),
-            ),
+                    .map(OperationArg::Index)
+                    .ok_or(OperationArgError::StatementNotFound)
+            }
         }
     }
 
@@ -268,7 +289,7 @@ impl MockMainPod {
         params: &Params,
         statements: &[Statement],
         input_operations: &[middleware::Operation],
-    ) -> Vec<Operation> {
+    ) -> Result<Vec<Operation>, OperationArgError> {
         let op_none = Self::operation_none(params);
 
         let mut operations = Vec::new();
@@ -278,11 +299,12 @@ impl MockMainPod {
             Self::pad_operation_args(params, &mut mid_args);
             let mut args = Vec::with_capacity(mid_args.len());
             for mid_arg in &mid_args {
-                args.push(Self::find_op_arg(statements, mid_arg));
+                let op_arg = Self::find_op_arg(statements, mid_arg)?;
+                args.push(op_arg)
             }
             operations.push(Operation(op.0, args));
         }
-        operations
+        Ok(operations)
     }
 
     // NOTE: In this implementation public statements are always copies from previous statements,
@@ -291,9 +313,7 @@ impl MockMainPod {
         params: &Params,
         statements: &[Statement],
         mut operations: Vec<Operation>,
-    ) -> Vec<Operation> {
-        let op_none = Self::operation_none(params);
-
+    ) -> Result<Vec<Operation>, OperationArgError> {
         let offset_public_statements = statements.len() - params.max_public_statements;
         operations.push(Operation(NativeOperation::NewEntry, vec![]));
         for i in 0..(params.max_public_statements - 1) {
@@ -302,15 +322,16 @@ impl MockMainPod {
                 Operation(NativeOperation::None, vec![])
             } else {
                 let mid_arg = middleware::OperationArg::Statement(st.clone());
+                let op_arg = Self::find_op_arg(statements, &mid_arg)?;
                 Operation(
                     NativeOperation::CopyStatement,
-                    vec![Self::find_op_arg(statements, &mid_arg)],
+                    vec![op_arg],
                 )
             };
             fill_pad(&mut op.1, OperationArg::None, params.max_operation_args);
             operations.push(op);
         }
-        operations
+        Ok(operations)
     }
 
     pub fn new(params: &Params, inputs: MainPodInputs) -> Result<Self> {
@@ -321,9 +342,9 @@ impl MockMainPod {
         // value=PodType::MockMainPod`
         let statements = Self::layout_statements(params, &inputs);
         let operations =
-            Self::process_private_statements_operations(params, &statements, inputs.operations);
+            Self::process_private_statements_operations(params, &statements, inputs.operations)?;
         let operations =
-            Self::process_public_statements_operations(params, &statements, operations);
+            Self::process_public_statements_operations(params, &statements, operations)?;
 
         let input_signed_pods = inputs
             .signed_pods
diff --git a/src/frontend.rs b/src/frontend.rs
index bd32684..1465c8a 100644
--- a/src/frontend.rs
+++ b/src/frontend.rs
@@ -613,4 +613,4 @@ pub mod tests {
 
         Ok(())
     }
-}
+}
\ No newline at end of file