--- a/truffle/com.oracle.truffle.api.vm/src/com/oracle/truffle/api/vm/TruffleVM.java	Tue Sep 15 12:10:50 2015 -0700
+++ b/truffle/com.oracle.truffle.api.vm/src/com/oracle/truffle/api/vm/TruffleVM.java	Tue Sep 15 14:49:24 2015 -0700
@@ -806,7 +806,9 @@
                     impl = LanguageCache.find(n, loader());
                     Instrumenter inst = TruffleVM.this.instrumenter;
                     final ASTProber prober = SPI.getDefaultASTProber(TruffleVM.this, impl.getClass());
-                    inst.registerASTProber(prober);
+                    if (prober != null) {
+                        inst.registerASTProber(prober);
+                    }
                     env = SPI.attachEnv(TruffleVM.this, impl, out, err, in);
                 } catch (Exception ex) {
                     throw new IllegalStateException("Cannot initialize " + getShortName() + " language with implementation " + n, ex);

--- a/truffle/com.oracle.truffle.api/src/com/oracle/truffle/api/instrument/Instrumenter.java	Tue Sep 15 12:10:50 2015 -0700
+++ b/truffle/com.oracle.truffle.api/src/com/oracle/truffle/api/instrument/Instrumenter.java	Tue Sep 15 14:49:24 2015 -0700
@@ -28,7 +28,10 @@
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 
 import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
 import com.oracle.truffle.api.TruffleLanguage;
@@ -65,7 +68,7 @@
         return visitor.source;
     }
 
-    private final List<ASTProber> astProbers = new ArrayList<>();
+    private final Set<ASTProber> astProbers = Collections.synchronizedSet(new LinkedHashSet<ASTProber>());
 
     private final List<ProbeListener> probeListeners = new ArrayList<>();
 
@@ -257,9 +260,13 @@
     }
 
     /**
-     * Enables instrumentation at selected nodes in all subsequently constructed ASTs.
+     * Enables instrumentation at selected nodes in all subsequently constructed ASTs. Ignored if
+     * the argument is already registered, runtime error if argument is {@code null}.
      */
     public void registerASTProber(ASTProber prober) {
+        if (prober == null) {
+            throw new IllegalArgumentException("Register non-null ASTProbers");
+        }
         astProbers.add(prober);
     }