diff --git a/substrate/state_machine/src/backend.rs b/substrate/state_machine/src/backend.rs
index 9f07f2d98c..5362d03be1 100644
--- a/substrate/state_machine/src/backend.rs
+++ b/substrate/state_machine/src/backend.rs
@@ -67,6 +67,18 @@ pub struct InMemory {
 	inner: MemoryState, // keeps all the state in memory.
 }
 
+#[cfg(test)]
+impl InMemory {
+	/// Create a new instance from a given storage map.
+	pub fn from(storage: ::std::collections::HashMap<Vec<u8>, Vec<u8>>) -> Self {
+		InMemory {
+			inner: MemoryState {
+				storage
+			}
+		}
+	}
+}
+
 impl Backend for InMemory {
 	type Error = Void;
 
diff --git a/substrate/state_machine/src/lib.rs b/substrate/state_machine/src/lib.rs
index e1e674a419..11ea5adbb5 100644
--- a/substrate/state_machine/src/lib.rs
+++ b/substrate/state_machine/src/lib.rs
@@ -218,7 +218,9 @@ pub fn execute<B: backend::Backend, Exec: CodeExecutor>(
 
 #[cfg(test)]
 mod tests {
-	use super::{OverlayedChanges, Externalities, TestExternalities};
+	use super::*;
+	use super::backend::InMemory;
+	use super::ext::Ext;
 
 	#[test]
 	fn overlayed_storage_works() {
@@ -264,4 +266,34 @@ mod tests {
 		ext.set_storage(b"con:aut:\x01\0\0\0".to_vec(), b"second".to_vec());
 		assert_eq!(ext.authorities(), Ok(vec![&b"first"[..], &b"second"[..]]));
 	}
+
+	macro_rules! map {
+		($( $name:expr => $value:expr ),*) => (
+			vec![ $( ( $name, $value ) ),* ].into_iter().collect()
+		)
+	}
+
+	#[test]
+	fn overlayed_storage_root_works() {
+		let mut backend = InMemory::from(map![
+			b"doe".to_vec() => b"reindeer".to_vec(),
+			b"dog".to_vec() => b"puppyXXX".to_vec(),
+			b"dogglesworth".to_vec() => b"catXXX".to_vec()
+		]);
+		let mut overlay = OverlayedChanges {
+			committed: MemoryState { storage: map![
+				b"dog".to_vec() => b"puppy".to_vec(),
+				b"dogglesworth".to_vec() => b"catYYY".to_vec()
+			], },
+			prospective: MemoryState { storage: map![
+				b"dogglesworth".to_vec() => b"cat".to_vec()
+			], },
+		};
+		let ext = Ext {
+			backend: &mut backend,
+			overlay: &mut overlay,
+		};
+		const ROOT: [u8; 32] = hex!("8aad789dff2f538bca5d8ea56e8abe10f4c7ba3a5dea95fea4cd6e7c3a1168d3");
+		assert_eq!(ext.storage_root(), ROOT);
+	}
 }