diff --git a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
index ccfc7879..833ab8d1 100644
--- a/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
+++ b/pkl-server/src/main/kotlin/org/pkl/server/ServerMessagePackDecoder.kt
@@ -130,5 +130,5 @@ class ServerMessagePackDecoder(unpacker: MessageUnpacker) : BaseMessagePackDecod
   }
 
   private fun unpackExternalReader(map: Map<Value, Value>): ExternalReader =
-    ExternalReader(unpackString(map, "executable"), unpackStringListOrNull(map, "arguments")!!)
+    ExternalReader(unpackString(map, "executable"), unpackStringListOrNull(map, "arguments"))
 }