diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/impl/report/JCLComparisonBuilder.java b/teavm-classlib/src/main/java/org/teavm/classlib/impl/report/JCLComparisonBuilder.java
index 20232184b..38e78a6f0 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/impl/report/JCLComparisonBuilder.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/impl/report/JCLComparisonBuilder.java
@@ -166,7 +166,7 @@ public class JCLComparisonBuilder {
             return false;
         }
         int slashIndex = name.lastIndexOf('/');
-        return packages.contains(name.substring(0, slashIndex));
+        return slashIndex >= 0 && packages.contains(name.substring(0, slashIndex));
     }
 
     private void compareClass(InputStream input) throws IOException {