diff --git a/truffle/src/main/java/org/jruby/truffle/stdlib/CoverageManager.java b/truffle/src/main/java/org/jruby/truffle/stdlib/CoverageManager.java
index ef48185c82c..074e720f700 100644
--- a/truffle/src/main/java/org/jruby/truffle/stdlib/CoverageManager.java
+++ b/truffle/src/main/java/org/jruby/truffle/stdlib/CoverageManager.java
@@ -63,7 +63,7 @@ public synchronized void setLineHasCode(LineLocation line) {
     }
 
     @CompilerDirectives.TruffleBoundary
-    public void enable() {
+    public synchronized void enable() {
         if (enabled) {
             return;
         }