diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/AliasAwareOutputExpression.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/AliasAwareOutputExpression.scala
index 910cbcf2210a0..1f2b1d0a585d6 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/AliasAwareOutputExpression.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/AliasAwareOutputExpression.scala
@@ -94,6 +94,16 @@ trait PartitioningPreservingUnaryExecNode extends UnaryExecNode
       kps: Seq[KeyedPartitioning]): LazyList[KeyedPartitioning] = {
     if (kps.isEmpty) return LazyList.empty
     val numPositions = kps.head.expressions.length
+    // The function assumes all input KPs share the same `partitionKeys`, which implies matching
+    // expression arity. This invariant is asserted by [[GroupPartitionsExec]] and is established
+    // by the constructors of [[PartitioningCollection]] feeding this method (a join's
+    // `PartitioningCollection(left.outputPartitioning, right.outputPartitioning)` combines KPs
+    // that have been aligned by [[EnsureRequirements]] to the same join keys). If the invariant
+    // is ever violated upstream, fail early with a clear message instead of throwing an opaque
+    // `IndexOutOfBoundsException` from `kp.expressions(i)` below.
+    assert(kps.tail.forall(_.expressions.length == numPositions),
+      s"All input KeyedPartitionings must share the same expression arity, " +
+        s"but got: ${kps.map(_.expressions.length).mkString(", ")}.")
 
     val alternativesPerPosition: IndexedSeq[LazyList[Expression]] =
       if (hasAlias) {
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/ProjectedOrderingAndPartitioningSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/ProjectedOrderingAndPartitioningSuite.scala
index db664b04ef08b..a38570924620a 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/ProjectedOrderingAndPartitioningSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/ProjectedOrderingAndPartitioningSuite.scala
@@ -586,6 +586,29 @@ class ProjectedOrderingAndPartitioningSuite
       case other => fail(s"Expected KeyedPartitioning, got $other")
     }
   }
+
+  test("SPARK-46367: mixed-arity KeyedPartitionings in input fail with a clear assertion") {
+    // The function assumes all input KPs share the same arity (the invariant asserted by
+    // `GroupPartitionsExec`). Without the assert below, indexing `kp.expressions(i)` for
+    // `i >= kp.expressions.length` would throw an opaque `IndexOutOfBoundsException`. The assert
+    // surfaces the real cause -- an upstream node violated the invariant -- so the bug can be
+    // fixed at the producer.
+    val x = AttributeReference("x", IntegerType)()
+    val y = AttributeReference("y", IntegerType)()
+    val keys2d = Seq(InternalRow(1, 1), InternalRow(2, 2))
+    val keys1d = Seq(InternalRow(1), InternalRow(2))
+    val child = DummyLeafExecWithPartitioning(
+      output = Seq(x, y),
+      partitioning = PartitioningCollection(Seq(
+        KeyedPartitioning(Seq(x, y), keys2d),
+        KeyedPartitioning(Seq(x), keys1d))))
+    val project = ProjectExec(Seq(x), child)
+    val e = intercept[AssertionError] {
+      project.outputPartitioning
+    }
+    assert(e.getMessage.contains("All input KeyedPartitionings must share the same expression " +
+      "arity"))
+  }
 }
 
 private case class DummyLeafExecWithPartitioning(