diff --git a/lib/src/store/memory.rs b/lib/src/store/memory.rs
index 9eff3d43..34490901 100644
--- a/lib/src/store/memory.rs
+++ b/lib/src/store/memory.rs
@@ -1,14 +1,12 @@
-use failure::Backtrace;
 use std::collections::BTreeMap;
 use std::collections::BTreeSet;
-use std::str::FromStr;
-use std::sync::PoisonError;
 use std::sync::RwLock;
 use std::sync::RwLockReadGuard;
 use std::sync::RwLockWriteGuard;
 use store::encoded::*;
 use store::numeric_encoder::*;
 use url::Url;
+use utils::MutexPoisonError;
 use Result;
 
 /// Memory based implementation of the `rudf::model::Dataset` trait.
@@ -49,20 +47,16 @@ pub type MemoryDataset = StoreDataset<MemoryStore>;
 pub type MemoryGraph = StoreDefaultGraph<MemoryStore>;
 
 pub struct MemoryStore {
-    id2str: RwLock<Vec<String>>,
-    str2id: RwLock<BTreeMap<String, u64>>,
+    string_store: MemoryStringStore,
     graph_indexes: RwLock<BTreeMap<EncodedTerm, MemoryGraphIndexes>>,
 }
 
 impl Default for MemoryStore {
     fn default() -> Self {
-        let new = Self {
-            id2str: RwLock::default(),
-            str2id: RwLock::default(),
+        Self {
+            string_store: MemoryStringStore::default(),
             graph_indexes: RwLock::default(),
-        };
-        new.set_first_strings().unwrap();
-        new
+        }
     }
 }
 
@@ -75,32 +69,15 @@ struct MemoryGraphIndexes {
 
 impl StringStore for MemoryStore {
     fn insert_str(&self, value: &str) -> Result<u64> {
-        let mut id2str = self.id2str.write().map_err(MemoryStorePoisonError::from)?;
-        let mut str2id = self.str2id.write().map_err(MemoryStorePoisonError::from)?;
-        let id = str2id.entry(value.to_string()).or_insert_with(|| {
-            let id = id2str.len() as u64;
-            id2str.push(value.to_string());
-            id
-        });
-        Ok(*id)
+        self.string_store.insert_str(value)
     }
 
     fn get_str(&self, id: u64) -> Result<String> {
-        let id2str = self.id2str.read().map_err(MemoryStorePoisonError::from)?;
-        if id2str.len() as u64 <= id {
-            Err(format_err!("value not found in the dictionary"))
-        } else {
-            Ok(id2str[id as usize].to_owned())
-        }
+        self.string_store.get_str(id)
     }
 
     fn get_url(&self, id: u64) -> Result<Url> {
-        let id2str = self.id2str.read().map_err(MemoryStorePoisonError::from)?;
-        if id2str.len() as u64 <= id {
-            Err(format_err!("value not found in the dictionary"))
-        } else {
-            Ok(Url::from_str(&id2str[id as usize])?)
-        }
+        self.string_store.get_url(id)
     }
 }
 
@@ -505,32 +482,12 @@ impl EncodedQuadsStore for MemoryStore {
 
 impl MemoryStore {
     fn graph_indexes(&self) -> Result<RwLockReadGuard<BTreeMap<EncodedTerm, MemoryGraphIndexes>>> {
-        Ok(self
-            .graph_indexes
-            .read()
-            .map_err(MemoryStorePoisonError::from)?)
+        Ok(self.graph_indexes.read().map_err(MutexPoisonError::from)?)
     }
 
     fn graph_indexes_mut(
         &self,
     ) -> Result<RwLockWriteGuard<BTreeMap<EncodedTerm, MemoryGraphIndexes>>> {
-        Ok(self
-            .graph_indexes
-            .write()
-            .map_err(MemoryStorePoisonError::from)?)
-    }
-}
-
-#[derive(Debug, Fail)]
-#[fail(display = "MemoryStore Mutex was poisoned")]
-pub struct MemoryStorePoisonError {
-    backtrace: Backtrace,
-}
-
-impl<T> From<PoisonError<T>> for MemoryStorePoisonError {
-    fn from(_: PoisonError<T>) -> Self {
-        Self {
-            backtrace: Backtrace::new(),
-        }
+        Ok(self.graph_indexes.write().map_err(MutexPoisonError::from)?)
     }
 }
diff --git a/lib/src/store/numeric_encoder.rs b/lib/src/store/numeric_encoder.rs
index 8585f864..4ccedd1b 100644
--- a/lib/src/store/numeric_encoder.rs
+++ b/lib/src/store/numeric_encoder.rs
@@ -7,10 +7,14 @@ use model::vocab::xsd;
 use model::*;
 use ordered_float::OrderedFloat;
 use rust_decimal::Decimal;
+use std::collections::BTreeMap;
 use std::io::Read;
 use std::io::Write;
 use std::str;
+use std::str::FromStr;
+use std::sync::RwLock;
 use url::Url;
+use utils::MutexPoisonError;
 use uuid::Uuid;
 use Result;
 
@@ -50,6 +54,53 @@ pub trait StringStore {
     }
 }
 
+pub struct MemoryStringStore {
+    id2str: RwLock<Vec<String>>,
+    str2id: RwLock<BTreeMap<String, u64>>,
+}
+
+impl Default for MemoryStringStore {
+    fn default() -> Self {
+        let new = Self {
+            id2str: RwLock::default(),
+            str2id: RwLock::default(),
+        };
+        new.set_first_strings().unwrap();
+        new
+    }
+}
+
+impl StringStore for MemoryStringStore {
+    fn insert_str(&self, value: &str) -> Result<u64> {
+        let mut id2str = self.id2str.write().map_err(MutexPoisonError::from)?;
+        let mut str2id = self.str2id.write().map_err(MutexPoisonError::from)?;
+        let id = str2id.entry(value.to_string()).or_insert_with(|| {
+            let id = id2str.len() as u64;
+            id2str.push(value.to_string());
+            id
+        });
+        Ok(*id)
+    }
+
+    fn get_str(&self, id: u64) -> Result<String> {
+        let id2str = self.id2str.read().map_err(MutexPoisonError::from)?;
+        if id2str.len() as u64 <= id {
+            Err(format_err!("value not found in the dictionary"))
+        } else {
+            Ok(id2str[id as usize].to_owned())
+        }
+    }
+
+    fn get_url(&self, id: u64) -> Result<Url> {
+        let id2str = self.id2str.read().map_err(MutexPoisonError::from)?;
+        if id2str.len() as u64 <= id {
+            Err(format_err!("value not found in the dictionary"))
+        } else {
+            Ok(Url::from_str(&id2str[id as usize])?)
+        }
+    }
+}
+
 const TYPE_DEFAULT_GRAPH_ID: u8 = 0;
 const TYPE_NAMED_NODE_ID: u8 = 1;
 const TYPE_BLANK_NODE_ID: u8 = 2;
@@ -652,48 +703,6 @@ impl<S: StringStore + Default> Default for Encoder<S> {
 }
 
 mod test {
-    use std::cell::RefCell;
-    use std::collections::BTreeMap;
-    use std::str::FromStr;
-    use store::numeric_encoder::*;
-
-    #[derive(Default)]
-    struct MemoryStringStore {
-        id2str: RefCell<Vec<String>>,
-        str2id: RefCell<BTreeMap<String, u64>>,
-    }
-
-    impl StringStore for MemoryStringStore {
-        fn insert_str(&self, value: &str) -> Result<u64> {
-            let mut str2id = self.str2id.borrow_mut();
-            let mut id2str = self.id2str.borrow_mut();
-            let id = str2id.entry(value.to_string()).or_insert_with(|| {
-                let id = id2str.len() as u64;
-                id2str.push(value.to_string());
-                id
-            });
-            Ok(*id)
-        }
-
-        fn get_str(&self, id: u64) -> Result<String> {
-            let id2str = self.id2str.borrow();
-            if id2str.len() as u64 <= id {
-                Err(format_err!("value not found in the dictionary"))
-            } else {
-                Ok(id2str[id as usize].to_owned())
-            }
-        }
-
-        fn get_url(&self, id: u64) -> Result<Url> {
-            let id2str = self.id2str.borrow();
-            if id2str.len() as u64 <= id {
-                Err(format_err!("value not found in the dictionary"))
-            } else {
-                Ok(Url::from_str(&id2str[id as usize])?)
-            }
-        }
-    }
-
     #[test]
     fn test_encoding() {
         use model::*;
diff --git a/lib/src/store/rocksdb.rs b/lib/src/store/rocksdb.rs
index f0d97d34..e7730f93 100644
--- a/lib/src/store/rocksdb.rs
+++ b/lib/src/store/rocksdb.rs
@@ -1,6 +1,5 @@
 use byteorder::ByteOrder;
 use byteorder::LittleEndian;
-use failure::Backtrace;
 use rocksdb::ColumnFamily;
 use rocksdb::DBRawIterator;
 use rocksdb::Options;
@@ -12,11 +11,11 @@ use std::path::Path;
 use std::str;
 use std::str::FromStr;
 use std::sync::Mutex;
-use std::sync::PoisonError;
 use store::encoded::EncodedQuadsStore;
 use store::encoded::StoreDataset;
 use store::numeric_encoder::*;
 use url::Url;
+use utils::MutexPoisonError;
 use Result;
 
 /// `rudf::model::Dataset` trait implementation based on the [RocksDB](https://rocksdb.org/) key-value store
@@ -94,7 +93,7 @@ impl StringStore for RocksDbStore {
             let id = self
                 .str_id_counter
                 .lock()
-                .map_err(RocksDBCounterMutexPoisonError::from)?
+                .map_err(MutexPoisonError::from)?
                 .get_and_increment(&self.db)? as u64;
             let id_bytes = to_bytes(id);
             let mut batch = WriteBatch::default();
@@ -549,20 +548,6 @@ fn to_bytes(int: u64) -> [u8; 8] {
     buf
 }
 
-#[derive(Debug, Fail)]
-#[fail(display = "RocksDBStore Mutex was poisoned")]
-pub struct RocksDBCounterMutexPoisonError {
-    backtrace: Backtrace,
-}
-
-impl<T> From<PoisonError<T>> for RocksDBCounterMutexPoisonError {
-    fn from(_: PoisonError<T>) -> Self {
-        Self {
-            backtrace: Backtrace::new(),
-        }
-    }
-}
-
 // TODO: very bad but I believe it is fine
 #[derive(Clone, Copy)]
 struct SendColumnFamily(ColumnFamily);
diff --git a/lib/src/utils.rs b/lib/src/utils.rs
index fafa5d92..9d994263 100644
--- a/lib/src/utils.rs
+++ b/lib/src/utils.rs
@@ -1,3 +1,6 @@
+use failure::Backtrace;
+use std::sync::PoisonError;
+
 pub trait Escaper {
     fn escape(&self) -> String;
 }
@@ -109,3 +112,17 @@ impl<K: 'static + Copy + Eq, V: 'static + Copy> StaticSliceMap<K, V> {
         None
     }
 }
+
+#[derive(Debug, Fail)]
+#[fail(display = "Mutex Mutex was poisoned")]
+pub struct MutexPoisonError {
+    backtrace: Backtrace,
+}
+
+impl<T> From<PoisonError<T>> for MutexPoisonError {
+    fn from(_: PoisonError<T>) -> Self {
+        Self {
+            backtrace: Backtrace::new(),
+        }
+    }
+}