diff --git a/scalac-scoverage-runtime/shared/src/main/scala/scoverage/Invoker.scala b/scalac-scoverage-runtime/shared/src/main/scala/scoverage/Invoker.scala
index 5ee0c5a7..f7d222bf 100644
--- a/scalac-scoverage-runtime/shared/src/main/scala/scoverage/Invoker.scala
+++ b/scalac-scoverage-runtime/shared/src/main/scala/scoverage/Invoker.scala
@@ -7,8 +7,12 @@ import scoverage.Platform._
 object Invoker {
 
   private val MeasurementsPrefix = "scoverage.measurements."
-  private val threadFiles = new ThreadLocal[ThreadSafeMap[String, FileWriter]]
-  private val ids = ThreadSafeMap.empty[(String, Int), Any]
+  private val threadFiles = new ThreadLocal[mutable.HashMap[String, FileWriter]]
+
+  // For each data directory we maintain a thread-safe set tracking the ids that we've already
+  // seen and recorded. We're using a map as a set, so we only care about its keys and can ignore
+  // its values.
+  private val dataDirToIds = ThreadSafeMap.empty[String, ThreadSafeMap[Int, Any]]
 
   /**
    * We record that the given id has been invoked by appending its id to the coverage
@@ -31,18 +35,27 @@ object Invoker {
     // times since for coverage we only care about 1 or more, (it just slows things down to
     // do it more than once), anything we can do to help is good. This helps especially with code
     // that is executed many times quickly, eg tight loops.
-    if (!ids.contains(dataDir, id)) {
+    if (!dataDirToIds.contains(dataDir)) {
+      // Guard against SI-7943: "TrieMap method getOrElseUpdate is not thread-safe".
+      dataDirToIds.synchronized {
+        if (!dataDirToIds.contains(dataDir)) {
+          dataDirToIds(dataDir) = ThreadSafeMap.empty[Int, Any]
+        }
+      }
+    }
+    val ids = dataDirToIds(dataDir)
+    if (!ids.contains(id)) {
       // Each thread writes to a separate measurement file, to reduce contention
       // and because file appends via FileWriter are not atomic on Windows.
       var files = threadFiles.get()
       if (files == null) {
-        files = ThreadSafeMap.empty[String, FileWriter]
+        files = mutable.HashMap.empty[String, FileWriter]
         threadFiles.set(files)
       }
       val writer = files.getOrElseUpdate(dataDir, new FileWriter(measurementFile(dataDir), true))
-      writer.append(id.toString + '\n').flush()
+      writer.append(Integer.toString(id)).append("\n").flush()
 
-      ids.put((dataDir, id), ())
+      ids.put(id, ())
     }
   }