diff --git a/src/interpreter/instructions/strings.rs b/src/interpreter/instructions/strings.rs
index 011cac9..d84da47 100644
--- a/src/interpreter/instructions/strings.rs
+++ b/src/interpreter/instructions/strings.rs
@@ -10,7 +10,7 @@ use crate::{
         Instruction,
     },
 };
-use std::cell::Cell;
+use std::{cell::Cell, convert::TryInto};
 
 executable_instruction!(
     string_lift_memory(instruction: Instruction) -> _ {
@@ -23,7 +23,6 @@ executable_instruction!(
             })?;
 
             let memory_index: u32 = 0;
-
             let memory = runtime
                 .wasm_instance
                 .memory(memory_index as usize)
@@ -34,8 +33,18 @@ executable_instruction!(
                     )
                 })?;
 
-            let pointer = to_native::<i32>(&inputs[0], instruction)? as usize;
-            let length = to_native::<i32>(&inputs[1], instruction)? as usize;
+            let pointer: usize = to_native::<i32>(&inputs[0], instruction)?.try_into().map_err(|_| {
+                InstructionError::new(
+                    instruction,
+                    InstructionErrorKind::NegativeValue { subject: "pointer" },
+                )
+            })?;
+            let length: usize = to_native::<i32>(&inputs[1], instruction)?.try_into().map_err(|_| {
+                InstructionError::new(
+                    instruction,
+                    InstructionErrorKind::NegativeValue { subject: "length" },
+                )
+            })?;
             let memory_view = memory.view();
 
             if length == 0 {
@@ -102,7 +111,12 @@ executable_instruction!(
 
             let string: String = to_native(&string, instruction)?;
             let string_bytes = string.as_bytes();
-            let string_length = string_bytes.len() as i32;
+            let string_length: i32 = string_bytes.len().try_into().map_err(|_| {
+                InstructionError::new(
+                    instruction,
+                    InstructionErrorKind::NegativeValue { subject: "string_length" },
+                )
+            })?;
 
             let outputs = allocator.call(&[InterfaceValue::I32(string_length)]).map_err(|_| {
                 InstructionError::new(
@@ -110,7 +124,12 @@ executable_instruction!(
                     InstructionErrorKind::LocalOrImportCall { function_index: allocator_index },
                 )
             })?;
-            let string_pointer: i32 = to_native(&outputs[0], instruction)?;
+            let string_pointer: u32 = to_native::<i32>(&outputs[0], instruction)?.try_into().map_err(|_| {
+                InstructionError::new(
+                    instruction,
+                    InstructionErrorKind::NegativeValue { subject: "string_pointer" },
+                )
+            })?;
 
             let memory_index: u32 = 0;
             let memory_view = instance
@@ -127,7 +146,7 @@ executable_instruction!(
                 memory_view[string_pointer as usize + nth].set(*byte);
             }
 
-            runtime.stack.push(InterfaceValue::I32(string_pointer));
+            runtime.stack.push(InterfaceValue::I32(string_pointer as i32));
             runtime.stack.push(InterfaceValue::I32(string_length));
 
             Ok(())