diff --git a/sdk/src/main/java/com/google/cloud/dataflow/sdk/coders/AvroCoder.java b/sdk/src/main/java/com/google/cloud/dataflow/sdk/coders/AvroCoder.java
index 91efb43f35..c5aa029531 100644
--- a/sdk/src/main/java/com/google/cloud/dataflow/sdk/coders/AvroCoder.java
+++ b/sdk/src/main/java/com/google/cloud/dataflow/sdk/coders/AvroCoder.java
@@ -680,7 +680,7 @@ private void checkArray(String context, TypeDescriptor<?> type, Schema schema) {
       } else {
         // If it was an unknown type encoded as an array, be conservative and assume
         // that we don't know anything about the order.
-        reportError(context, "encoding %s as an ARRAY was unexpected");
+        reportError(context, "encoding %s as an ARRAY was unexpected", type);
         return;
       }