diff --git a/.gitignore b/.gitignore
index 81c1a16d62..7bdb2237b4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,6 +17,9 @@ tests/lenses/rust_wasm32_remove/pkg
 tests/lenses/rust_wasm32_copy/Cargo.lock
 tests/lenses/rust_wasm32_copy/target
 tests/lenses/rust_wasm32_copy/pkg
+tests/lenses/rust_wasm32_prepend/Cargo.lock
+tests/lenses/rust_wasm32_prepend/target
+tests/lenses/rust_wasm32_prepend/pkg
 
 # Ignore OS X metadata files.
 .history
diff --git a/tests/lenses/Makefile b/tests/lenses/Makefile
index 7370a04b80..57807e10b2 100644
--- a/tests/lenses/Makefile
+++ b/tests/lenses/Makefile
@@ -2,3 +2,4 @@ build:
 	cargo build --target wasm32-unknown-unknown --manifest-path "./rust_wasm32_set_default/Cargo.toml"
 	cargo build --target wasm32-unknown-unknown --manifest-path "./rust_wasm32_remove/Cargo.toml"
 	cargo build --target wasm32-unknown-unknown --manifest-path "./rust_wasm32_copy/Cargo.toml"
+	cargo build --target wasm32-unknown-unknown --manifest-path "./rust_wasm32_prepend/Cargo.toml"
diff --git a/tests/lenses/rust_wasm32_prepend/Cargo.toml b/tests/lenses/rust_wasm32_prepend/Cargo.toml
new file mode 100644
index 0000000000..7038a68b9c
--- /dev/null
+++ b/tests/lenses/rust_wasm32_prepend/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "rust-wasm32-prepend"
+version = "0.1.0"
+edition = "2018"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0.87"
+lens_sdk = "^0.5"
diff --git a/tests/lenses/rust_wasm32_prepend/src/lib.rs b/tests/lenses/rust_wasm32_prepend/src/lib.rs
new file mode 100644
index 0000000000..30dcab58f8
--- /dev/null
+++ b/tests/lenses/rust_wasm32_prepend/src/lib.rs
@@ -0,0 +1,111 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+use std::collections::HashMap;
+use std::sync::RwLock;
+use std::error::Error;
+use std::{fmt, error};
+use serde::Deserialize;
+use lens_sdk::StreamOption;
+use lens_sdk::option::StreamOption::{Some, None, EndOfStream};
+
+#[link(wasm_import_module = "lens")]
+extern "C" {
+    fn next() -> *mut u8;
+}
+
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+enum ModuleError {
+    ParametersNotSetError,
+}
+
+impl error::Error for ModuleError { }
+
+impl fmt::Display for ModuleError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match &*self {
+            ModuleError::ParametersNotSetError => f.write_str("Parameters have not been set."),
+        }
+    }
+}
+
+#[derive(Deserialize, Clone)]
+pub struct Parameters {
+    pub values: Vec<HashMap<String, String>>,
+}
+
+static PARAMETERS: RwLock<StreamOption<Parameters>> = RwLock::new(None);
+static PARAM_INDEX: RwLock<usize> = RwLock::new(0);
+
+#[no_mangle]
+pub extern fn alloc(size: usize) -> *mut u8 {
+    lens_sdk::alloc(size)
+}
+
+#[no_mangle]
+pub extern fn set_param(ptr: *mut u8) -> *mut u8 {
+    match try_set_param(ptr) {
+        Ok(_) => lens_sdk::nil_ptr(),
+        Err(e) => lens_sdk::to_mem(lens_sdk::ERROR_TYPE_ID, &e.to_string().as_bytes())
+    }
+}
+
+fn try_set_param(ptr: *mut u8) -> Result<(), Box<dyn Error>> {
+    let parameter = lens_sdk::try_from_mem::<Parameters>(ptr)?
+        .ok_or(ModuleError::ParametersNotSetError)?
+        .clone();
+
+    let mut dst = PARAMETERS.write()?;
+    *dst = Some(parameter);
+    Ok(())
+}
+
+#[no_mangle]
+pub extern fn transform() -> *mut u8 {
+    match try_transform() {
+        Ok(o) => match o {
+            Some(result_json) => lens_sdk::to_mem(lens_sdk::JSON_TYPE_ID, &result_json),
+            None => lens_sdk::nil_ptr(),
+            EndOfStream => lens_sdk::to_mem(lens_sdk::EOS_TYPE_ID, &[]),
+        },
+        Err(e) => lens_sdk::to_mem(lens_sdk::ERROR_TYPE_ID, &e.to_string().as_bytes())
+    }
+}
+
+fn try_transform() -> Result<StreamOption<Vec<u8>>, Box<dyn Error>> {
+    let params = PARAMETERS.read()?
+        .clone()
+        .ok_or(ModuleError::ParametersNotSetError)?
+        .clone();
+
+    let param_index = PARAM_INDEX.read()?
+        .clone();
+
+    if param_index < params.values.len() {
+        let result = &params.values[param_index];
+        let result_json = serde_json::to_vec(&result)?;
+
+        let mut dst = PARAM_INDEX.write()?;
+        *dst = param_index+1;
+        return Ok(Some(result_json))
+    }
+
+    // Note: The following is a very unperformant, but simple way of yielding the input documents,
+    // as this module is only used for testing, this is preferred.
+
+    let ptr = unsafe { next() };
+    let input = match lens_sdk::try_from_mem::<HashMap<String, serde_json::Value>>(ptr)? {
+        Some(v) => v,
+        // Implementations of `transform` are free to handle nil however they like. In this
+        // implementation we chose to return nil given a nil input.
+        None => return Ok(None),
+        EndOfStream => return Ok(EndOfStream)
+    };
+
+    let result = input.clone();
+
+    let result_json = serde_json::to_vec(&result)?;
+    lens_sdk::free_transport_buffer(ptr)?;
+    Ok(Some(result_json))
+}
diff --git a/tests/lenses/utils.go b/tests/lenses/utils.go
index 132c7d33c4..97619a6124 100644
--- a/tests/lenses/utils.go
+++ b/tests/lenses/utils.go
@@ -44,6 +44,15 @@ var CopyModulePath string = getPathRelativeToProjectRoot(
 	"/tests/lenses/rust_wasm32_copy/target/wasm32-unknown-unknown/debug/rust_wasm32_copy.wasm",
 )
 
+// PrependModulePath is the path to the `Prepend` lens module compiled to wasm.
+//
+// The module has one parameter:
+//   - `values` is an array of `map[string]string`s, the module will yield these documents before
+//     any documents fed to it (from Defra).
+var PrependModulePath string = getPathRelativeToProjectRoot(
+	"/tests/lenses/rust_wasm32_prepend/target/wasm32-unknown-unknown/debug/rust_wasm32_prepend.wasm",
+)
+
 func getPathRelativeToProjectRoot(relativePath string) string {
 	_, filename, _, _ := runtime.Caller(0)
 	root := path.Dir(path.Dir(path.Dir(filename)))