diff --git a/core/src/main/java/io/substrait/expression/Expression.java b/core/src/main/java/io/substrait/expression/Expression.java
index 9fc719f35..aa9e69148 100644
--- a/core/src/main/java/io/substrait/expression/Expression.java
+++ b/core/src/main/java/io/substrait/expression/Expression.java
@@ -940,6 +940,7 @@ public static AggregationInvocation fromProto(AggregateFunction.AggregationInvoc
   }
 
   enum AggregationPhase {
+    UNSPECIFIED(io.substrait.proto.AggregationPhase.AGGREGATION_PHASE_UNSPECIFIED),
     INITIAL_TO_INTERMEDIATE(
         io.substrait.proto.AggregationPhase.AGGREGATION_PHASE_INITIAL_TO_INTERMEDIATE),
     INTERMEDIATE_TO_INTERMEDIATE(
diff --git a/spark/src/main/scala/io/substrait/spark/expression/ToAggregateFunction.scala b/spark/src/main/scala/io/substrait/spark/expression/ToAggregateFunction.scala
index 9e959e47e..978287d93 100644
--- a/spark/src/main/scala/io/substrait/spark/expression/ToAggregateFunction.scala
+++ b/spark/src/main/scala/io/substrait/spark/expression/ToAggregateFunction.scala
@@ -74,6 +74,8 @@ object ToAggregateFunction {
     case other => throw new UnsupportedOperationException(s"not currently supported: $other.")
   }
   def toSpark(phase: SExpression.AggregationPhase): AggregateMode = phase match {
+    case SExpression.AggregationPhase.UNSPECIFIED =>
+      Final // UNSPECIFIED implies INTERMEDIATE_TO_RESULT
     case SExpression.AggregationPhase.INITIAL_TO_INTERMEDIATE => Partial
     case SExpression.AggregationPhase.INTERMEDIATE_TO_INTERMEDIATE => PartialMerge
     case SExpression.AggregationPhase.INTERMEDIATE_TO_RESULT => Final