1 files changed, 10 insertions, 4 deletions
diff --git a/src/main/scala/chisel3/internal/firrtl/Emitter.scala b/src/main/scala/chisel3/internal/firrtl/Emitter.scala
index 09984722..26ccc09d 100644
--- a/src/main/scala/chisel3/internal/firrtl/Emitter.scala
+++ b/src/main/scala/chisel3/internal/firrtl/Emitter.scala
@@ -159,10 +159,16 @@ private class Emitter(circuit: Circuit) {
     * alternative-free statements reset the indent level to the
     * enclosing block upon emission.
     */
-  private def processWhens(cmds: Seq[Command]):
-  Seq[Command] = { cmds.zip(cmds.tail).map({ case (a: WhenEnd, b:
-  AltBegin) => a.copy(hasAlt = true) case (a, b) => a }) ++
-  cmds.lastOption }
+  private def processWhens(cmds: Seq[Command]): Seq[Command] = {
+    if (cmds.isEmpty) {
+      Seq.empty
+    } else {
+      cmds.zip(cmds.tail).map{
+        case (a: WhenEnd, b: AltBegin) => a.copy(hasAlt = true)
+        case (a, b) => a
+      } ++ cmds.lastOption
+    }
+  }
 
   private var indentLevel = 0
   private def newline = "\n" + ("  " * indentLevel)