diff --git a/substrate/core/sr-std/with_std.rs b/substrate/core/sr-std/with_std.rs
index 443fea1a61..0e809e67d8 100644
--- a/substrate/core/sr-std/with_std.rs
+++ b/substrate/core/sr-std/with_std.rs
@@ -29,7 +29,6 @@ pub use std::ops;
 pub use std::ptr;
 pub use std::rc;
 pub use std::slice;
-pub use std::string;
 pub use std::vec;
 pub use std::result;
 
diff --git a/substrate/core/sr-std/without_std.rs b/substrate/core/sr-std/without_std.rs
index bfa9020504..981c02c025 100644
--- a/substrate/core/sr-std/without_std.rs
+++ b/substrate/core/sr-std/without_std.rs
@@ -47,7 +47,6 @@ mod __impl {
 pub use alloc::boxed;
 pub use alloc::rc;
 pub use alloc::vec;
-pub use alloc::string;
 pub use core::borrow;
 pub use core::cell;
 pub use core::clone;
@@ -63,6 +62,9 @@ pub use core::ops;
 pub use core::ptr;
 pub use core::slice;
 pub use core::result;
+// We are trying to avoid certain things here, such as `core::string`
+// (if you need `String` you most probably doing something wrong, since
+// runtime doesn't require anything human readable).
 
 pub mod collections {
 	pub use alloc::collections::btree_map;
diff --git a/substrate/srml/contract/src/vm/env_def/macros.rs b/substrate/srml/contract/src/vm/env_def/macros.rs
index 40651749eb..628da0b56d 100644
--- a/substrate/srml/contract/src/vm/env_def/macros.rs
+++ b/substrate/srml/contract/src/vm/env_def/macros.rs
@@ -280,6 +280,6 @@ mod tests {
 		);
 
 		let env = init_env::<MockExt>();
-		assert!(env.funcs.get("ext_gas").is_some());
+		assert!(env.funcs.get(&b"ext_gas"[..]).is_some());
 	}
 }
diff --git a/substrate/srml/contract/src/vm/env_def/mod.rs b/substrate/srml/contract/src/vm/env_def/mod.rs
index 3b208ebf9b..bd2c98668c 100644
--- a/substrate/srml/contract/src/vm/env_def/mod.rs
+++ b/substrate/srml/contract/src/vm/env_def/mod.rs
@@ -18,7 +18,6 @@ use super::{SpecialTrap, BalanceOf, CreateReceipt, Ext, GasMeterResult, Runtime}
 use codec::{Encode, Decode};
 use parity_wasm::elements::{FunctionType, ValueType};
 use rstd::prelude::*;
-use rstd::string::String;
 use rstd::collections::btree_map::BTreeMap;
 use runtime_primitives::traits::{As, CheckedMul};
 use sandbox::{self, TypedValue};
@@ -75,7 +74,7 @@ impl ConvertibleToWasm for u64 {
 /// which can be imported and called by the module.
 pub(crate) struct HostFunctionSet<E: Ext> {
 	/// Functions which defined in the environment.
-	pub funcs: BTreeMap<String, HostFunction<E>>,
+	pub funcs: BTreeMap<Vec<u8>, HostFunction<E>>,
 }
 impl<E: Ext> HostFunctionSet<E> {
 	pub fn new() -> Self {
diff --git a/substrate/srml/contract/src/vm/prepare.rs b/substrate/srml/contract/src/vm/prepare.rs
index b15f9cbc3a..7c1b7c3d41 100644
--- a/substrate/srml/contract/src/vm/prepare.rs
+++ b/substrate/srml/contract/src/vm/prepare.rs
@@ -138,7 +138,7 @@ impl<'a, T: Trait> ContractModule<'a, T> {
 
 			let ext_func = env
 				.funcs
-				.get(import.field())
+				.get(import.field().as_bytes())
 				.ok_or_else(|| Error::Instantiate)?;
 			if !ext_func.func_type_matches(func_ty) {
 				return Err(Error::Instantiate);