--- a/src/StreamletAttributeFinder.h	Sat Jan 25 21:28:37 2020 +0100
+++ b/src/StreamletAttributeFinder.h	Sun Jan 26 21:35:02 2020 +0100
@@ -143,6 +143,7 @@
 						SubProcess::Message m = subProcess->read();
 						if (m.code == StreamletMsg::OUTPUT_ATTRIBUTE_METADATA) metadata.push_back({m.parameters[0], writer->toTypeId(m.parameters[1])});
 						else if (m.code == StreamletMsg::WAITING_FOR_INPUT_ATTRIBUTES) break;
+						else throw RelpipeWriterException(L"Protocol violation from exec sub-process while reading output attribute metadata. Expected OUTPUT_ATTRIBUTE_METADATA or WAITING_FOR_INPUT_ATTRIBUTES but got: " + m.toString());
 					}
 
 					cachedMetadata[field.id] = metadata;