--- a/src/StreamletAttributeFinder.h	Tue Jan 28 23:35:25 2020 +0100
+++ b/src/StreamletAttributeFinder.h	Wed Jan 29 00:58:37 2020 +0100
@@ -82,6 +82,16 @@
 		}
 	}
 
+	void writeAttribute(RelationalWriter* writer, TypeId typeId, SubProcess::Message* m) {
+		if (m->parameters[1] == L"true") {
+			if (typeId == TypeId::BOOLEAN) writer->writeAttribute(L"false");
+			else if (typeId == TypeId::INTEGER)writer->writeAttribute(L"0");
+			else writer->writeAttribute(L""); // TODO: write acruall null values (when supported)
+		} else {
+			writer->writeAttribute(m->parameters[0]);
+		}
+	}
+
 protected:
 
 	void startFile(const fs::path& file, const string& fileRaw, bool exists) override {
@@ -98,7 +108,7 @@
 		if (field.group == RequestedField::GROUP_STREAMLET) {
 			for (auto metadata : cachedMetadata[field.id]) {
 				SubProcess::Message m = subProcesses[field.id]->read();
-				if (m.code == StreamletMsg::OUTPUT_ATTRIBUTE) writer->writeAttribute(m.parameters[0]);
+				if (m.code == StreamletMsg::OUTPUT_ATTRIBUTE) writeAttribute(writer, metadata.typeId, &m);
 				else throw RelpipeWriterException(L"Protocol violation from exec sub-process while reading: „" + metadata.attributeName + L"“. Expected OUTPUT_ATTRIBUTE but got: " + m.toString());
 			}
 

--- a/streamlet-examples/cloc	Tue Jan 28 23:35:25 2020 +0100
+++ b/streamlet-examples/cloc	Wed Jan 29 00:58:37 2020 +0100
@@ -64,9 +64,8 @@
 	for (( i=0; i<${#clocFields[@]}; i++)); do
 		value="${!clocFields[$i]}";
 
-		if [[ "x$files" == "x1" ]]; then                                isNull="false";
-		elif [[ "x${clocFields[$i]}" == "xlanguage" ]]; then value="";  isNull="true";
-		else                                                 value="0"; isNull="true"; fi
+		if [[ "x$files" == "x1" ]]; then isNull="false";
+		else                             isNull="true"; fi
 
 		send OUTPUT_ATTRIBUTE "$value"    "$isNull";
 	done

--- a/streamlet-examples/exiv2	Tue Jan 28 23:35:25 2020 +0100
+++ b/streamlet-examples/exiv2	Wed Jan 29 00:58:37 2020 +0100
@@ -68,7 +68,7 @@
 		else                                                                               isNull="false";
 		fi
 
-		if ( [[ "x${streamletFields[$i]}" == "xImage height" ]] || [[ "x${streamletFields[$i]}" == "xImage width" ]] ) && [[ ! "$value" =~ ^[0-9]+$ ]]; then value="0"; isNull="true"; fi
+		if ( [[ "x${streamletFields[$i]}" == "xImage height" ]] || [[ "x${streamletFields[$i]}" == "xImage width" ]] ) && [[ ! "$value" =~ ^[0-9]+$ ]]; then isNull="true"; fi
 
 		send OUTPUT_ATTRIBUTE "$value"    "$isNull";
 	done

--- a/streamlet-examples/lines_count	Tue Jan 28 23:35:25 2020 +0100
+++ b/streamlet-examples/lines_count	Wed Jan 29 00:58:37 2020 +0100
@@ -30,7 +30,6 @@
 
 processMessage_WAITING_FOR_OUTPUT_ATTRIBUTES() {
 	if [[ -d "$currentFile" ]]; then
-		value="0";
 		isNull="true";
 	else
 		value=$(wc -l "$currentFile" | cut -d" " -f1);

--- a/streamlet-examples/pdfinfo	Tue Jan 28 23:35:25 2020 +0100
+++ b/streamlet-examples/pdfinfo	Wed Jan 29 00:58:37 2020 +0100
@@ -58,9 +58,8 @@
 	for (( i=0; i<${#pdfFields[@]}; i++)); do
 		value="$(echo "$pdfInfo" | grep -P "^\Q${pdfFields[$i]}\E:" | sed -E 's/[^:]+:\s+(.*)/\1/g' | tr -d '\n';)"; # the field name must not contain "\E"
 
-		if  ([[ ! "x$pdfValid" == "x0" ]] || [[ "x$value" == "x" ]]) && [[ "x${pdfFields[$i]}" == "xPages" ]]; then value="0"; isNull="true";
-		elif [[ ! "x$pdfValid" == "x0" ]];                                                                     then value="";  isNull="true";
-		else                                                                                                                   isNull="false";
+		if [[ ! "x$pdfValid" == "x0" ]] || [[ "x$value" == "x" ]]; then isNull="true";
+		else                                                            isNull="false";
 		fi
 
 		send OUTPUT_ATTRIBUTE "$value"    "$isNull";

--- a/streamlet-examples/xpath.cpp	Tue Jan 28 23:35:25 2020 +0100
+++ b/streamlet-examples/xpath.cpp	Wed Jan 29 00:58:37 2020 +0100
@@ -62,13 +62,6 @@
 		else return STRING;
 	}
 
-	// TODO: should not be done in particular streamlets but in the worker
-	std::wstring toNullValue(std::wstring type) {
-		if (type == BOOLEAN) return L"false";
-		else if (type == INTEGER) return L"0";
-		else return L"";
-	}
-
 	class XPathAttribute {
 	public:
 
@@ -118,30 +111,22 @@
 					result = root->eval_to_boolean(xpath, ns) ? L"true" : L"false";
 				} else if (xpathAttribute.mode == Mode::LINE_NUMBER) {
 					xmlpp::NodeSet attributeNodes = root->find(xpath, ns);
-					if (attributeNodes.size()) {
-						result = std::to_wstring(attributeNodes[0]->get_line());
-					} else {
-						result = L"0";
-						isNull = true;
-					}
+					if (attributeNodes.size()) result = std::to_wstring(attributeNodes[0]->get_line());
+					else isNull = true;
 				} else if (xpathAttribute.mode == Mode::XPATH) {
 					xmlpp::NodeSet attributeNodes = root->find(xpath, ns);
-					if (attributeNodes.size()) {
-						result = convertor.from_bytes(attributeNodes[0]->get_path());
-					} else {
-						result = L"";
-						isNull = true;
-					}
+					if (attributeNodes.size()) result = convertor.from_bytes(attributeNodes[0]->get_path());
+					else isNull = true;
 				} else if (xpathAttribute.mode == Mode::RAW_XML) {
 					throw std::logic_error("Raw XML mode is not yet implemented."); // TODO: implement also RAW_XML
 				} else {
 					throw std::logic_error("Unsupported mode."); // should never happer
 				}
 
-				oa.push_back({result, false});
+				oa.push_back({result, isNull});
 			}
 		} catch (xmlpp::parse_error& e) {
-			for (XPathAttribute xpathAttribute : xpathAttributes) oa.push_back({toNullValue(toType(xpathAttribute.mode)), true});
+			for (XPathAttribute xpathAttribute : xpathAttributes) oa.push_back({L"", true});
 			// invalid XML → xmlpp::parse_error → just skip this file
 			// invalid XPath → xmlpp::exception → failure
 		}