diff --git a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
index 50a325f..2fd9ef9 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java
@@ -828,7 +828,8 @@
      */
     public Builder addArtifact(Artifact artifact) {
       Preconditions.checkNotNull(artifact);
-      Preconditions.checkArgument(!artifact.isMiddlemanArtifact());
+      Preconditions.checkArgument(
+          !artifact.isMiddlemanArtifact(), "unexpected middleman artifact: %s", artifact);
       artifactsBuilder.add(artifact);
       return this;
     }