diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/async/CircuitBreaker.java b/src/main/java/ch/ethz/seb/sebserver/gbl/async/CircuitBreaker.java
index 9d214d52..3f13ea72 100644
--- a/src/main/java/ch/ethz/seb/sebserver/gbl/async/CircuitBreaker.java
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/async/CircuitBreaker.java
@@ -121,7 +121,7 @@ public final class CircuitBreaker<T> {
         return this.lastSuccessTime;
     }
 
-    public Result<T> protectedRun(final Supplier<T> supplier) {
+    public synchronized Result<T> protectedRun(final Supplier<T> supplier) {
         final long currentTime = System.currentTimeMillis();
 
         if (log.isDebugEnabled()) {
diff --git a/src/main/java/ch/ethz/seb/sebserver/gbl/async/MemoizingCircuitBreaker.java b/src/main/java/ch/ethz/seb/sebserver/gbl/async/MemoizingCircuitBreaker.java
index 3c78c65c..5a236a5d 100644
--- a/src/main/java/ch/ethz/seb/sebserver/gbl/async/MemoizingCircuitBreaker.java
+++ b/src/main/java/ch/ethz/seb/sebserver/gbl/async/MemoizingCircuitBreaker.java
@@ -124,7 +124,7 @@ public final class MemoizingCircuitBreaker<T> implements Supplier<Result<T>> {
     }
 
     @Override
-    public Result<T> get() {
+    public synchronized Result<T> get() {
         final Result<T> result = this.delegate.protectedRun(this.supplier);
         if (result.hasError()) {
             if (this.memoizing && this.cached != null) {