diff --git a/lang/java/avro/src/main/java/org/apache/avro/UnresolvedUnionException.java b/lang/java/avro/src/main/java/org/apache/avro/UnresolvedUnionException.java
index 8f25c2cc9a8..1e11a4967af 100644
--- a/lang/java/avro/src/main/java/org/apache/avro/UnresolvedUnionException.java
+++ b/lang/java/avro/src/main/java/org/apache/avro/UnresolvedUnionException.java
@@ -24,13 +24,14 @@ public class UnresolvedUnionException extends AvroRuntimeException {
   private Schema unionSchema;
 
   public UnresolvedUnionException(Schema unionSchema, Object unresolvedDatum) {
-    super("Not in union " + unionSchema + ": " + unresolvedDatum);
+    super("Not in union " + unionSchema + ": " + datumTypeDescription(unresolvedDatum));
     this.unionSchema = unionSchema;
     this.unresolvedDatum = unresolvedDatum;
   }
 
   public UnresolvedUnionException(Schema unionSchema, Schema.Field field, Object unresolvedDatum) {
-    super("Not in union " + unionSchema + ": " + unresolvedDatum + " (field=" + field.name() + ")");
+    super(
+        "Not in union " + unionSchema + ": " + datumTypeDescription(unresolvedDatum) + " (field=" + field.name() + ")");
     this.unionSchema = unionSchema;
     this.unresolvedDatum = unresolvedDatum;
   }
@@ -42,4 +43,8 @@ public Object getUnresolvedDatum() {
   public Schema getUnionSchema() {
     return unionSchema;
   }
+
+  private static String datumTypeDescription(Object datum) {
+    return datum == null ? "null" : datum.getClass().getName();
+  }
 }
diff --git a/lang/java/avro/src/test/java/org/apache/avro/generic/TestGenericDatumWriter.java b/lang/java/avro/src/test/java/org/apache/avro/generic/TestGenericDatumWriter.java
index dc3661f467b..03123ffcf86 100644
--- a/lang/java/avro/src/test/java/org/apache/avro/generic/TestGenericDatumWriter.java
+++ b/lang/java/avro/src/test/java/org/apache/avro/generic/TestGenericDatumWriter.java
@@ -56,7 +56,7 @@ void unionUnresolvedExceptionExplicitWhichField() throws IOException {
       new GenericDatumWriter<>(s).write(r, EncoderFactory.get().jsonEncoder(s, bao));
       fail();
     } catch (final UnresolvedUnionException uue) {
-      assertEquals("Not in union [\"null\",\"string\"]: 100 (field=f)", uue.getMessage());
+      assertEquals("Not in union [\"null\",\"string\"]: java.lang.Integer (field=f)", uue.getMessage());
     }
   }