diff --git a/extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/input/external/ExternalInputSpecSlicer.java b/extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/input/external/ExternalInputSpecSlicer.java
index e745b2ac071..8583162cd0e 100644
--- a/extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/input/external/ExternalInputSpecSlicer.java
+++ b/extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/input/external/ExternalInputSpecSlicer.java
@@ -161,7 +161,7 @@ public class ExternalInputSpecSlicer implements InputSpecSlicer
         ((SplittableInputSource<?>) splittableInputSource.withSplit((InputSplit) split))
             .createSplits(spec.getInputFormat(), FilePerSplitHintSpec.INSTANCE)
             .map(subSplit -> splittableInputSource.withSplit((InputSplit) subSplit))
-            .forEach(subSources::add);
+            .forEach(s -> ((List) subSources).add(s));
       }
 
       if (subSources.isEmpty()) {