diff --git a/eth2/utils/slot_clock/src/testing_slot_clock.rs b/eth2/utils/slot_clock/src/testing_slot_clock.rs
index de1d7ddb3..330d47f1a 100644
--- a/eth2/utils/slot_clock/src/testing_slot_clock.rs
+++ b/eth2/utils/slot_clock/src/testing_slot_clock.rs
@@ -1,11 +1,12 @@
 use super::SlotClock;
+use std::sync::RwLock;
 
 #[derive(Debug, PartialEq)]
 pub enum Error {}
 
 /// Determines the present slot based upon the present system time.
 pub struct TestingSlotClock {
-    slot: u64,
+    slot: RwLock<u64>,
 }
 
 impl TestingSlotClock {
@@ -13,11 +14,13 @@ impl TestingSlotClock {
     ///
     /// Returns an Error if `slot_duration_seconds == 0`.
     pub fn new(slot: u64) -> TestingSlotClock {
-        TestingSlotClock { slot }
+        TestingSlotClock {
+            slot: RwLock::new(slot),
+        }
     }
 
-    pub fn set_slot(&mut self, slot: u64) {
-        self.slot = slot;
+    pub fn set_slot(&self, slot: u64) {
+        *self.slot.write().expect("TestingSlotClock poisoned.") = slot;
     }
 }
 
@@ -25,7 +28,8 @@ impl SlotClock for TestingSlotClock {
     type Error = Error;
 
     fn present_slot(&self) -> Result<Option<u64>, Error> {
-        Ok(Some(self.slot))
+        let slot = *self.slot.read().expect("TestingSlotClock poisoned.");
+        Ok(Some(slot))
     }
 }
 
@@ -35,7 +39,7 @@ mod tests {
 
     #[test]
     fn test_slot_now() {
-        let mut clock = TestingSlotClock::new(10);
+        let clock = TestingSlotClock::new(10);
         assert_eq!(clock.present_slot(), Ok(Some(10)));
         clock.set_slot(123);
         assert_eq!(clock.present_slot(), Ok(Some(123)));