diff --git a/src/kdefrontend/spreadsheet/FunctionValuesDialog.h b/src/kdefrontend/spreadsheet/FunctionValuesDialog.h
--- a/src/kdefrontend/spreadsheet/FunctionValuesDialog.h
+++ b/src/kdefrontend/spreadsheet/FunctionValuesDialog.h
@@ -49,6 +49,7 @@
 	explicit FunctionValuesDialog(Spreadsheet* s, QWidget* parent = nullptr);
 	~FunctionValuesDialog() override;
 	void setColumns(QVector<Column*>);
+	bool validVariableName(QLineEdit *le);
 
 private:
 	Ui::FunctionValuesWidget ui;
diff --git a/src/kdefrontend/spreadsheet/FunctionValuesDialog.cpp b/src/kdefrontend/spreadsheet/FunctionValuesDialog.cpp
--- a/src/kdefrontend/spreadsheet/FunctionValuesDialog.cpp
+++ b/src/kdefrontend/spreadsheet/FunctionValuesDialog.cpp
@@ -31,6 +31,7 @@
 #include "backend/core/Project.h"
 #include "backend/lib/macros.h"
 #include "backend/spreadsheet/Spreadsheet.h"
+#include "backend/gsl/ExpressionParser.h"
 #include "commonfrontend/widgets/TreeViewComboBox.h"
 #include "kdefrontend/widgets/ConstantsWidget.h"
 #include "kdefrontend/widgets/FunctionsWidget.h"
@@ -158,6 +159,25 @@
 	checkValues();
 }
 
+bool FunctionValuesDialog::validVariableName(QLineEdit* le) {
+
+	if (ExpressionParser::getInstance()->constants().indexOf(le->text()) != -1) {
+		le->setStyleSheet("QLineEdit{background: red;}");
+		le->setToolTip(i18n("Provided variable name is already reserved for a name of a constant. Please use another name."));
+		return false;
+	}
+
+	if (ExpressionParser::getInstance()->functions().indexOf(le->text()) != -1) {
+		le->setStyleSheet("QLineEdit{background: red;}");
+		le->setToolTip(i18n("Provided variable name is already reserved for a name of a function. Please use another name."));
+		return false;
+	}
+
+	le->setStyleSheet(QString());
+	le->setToolTip("");
+	return true;
+}
+
 /*!
 	check the user input and enables/disables the Ok-button depending on the correctness of the input
  */
@@ -179,6 +199,11 @@
 			m_okButton->setEnabled(false);
 			return;
 		}
+
+		if (!validVariableName(m_variableNames[i])) {
+			m_okButton->setEnabled(false);
+			return;
+		}
 /*		Column* column = dynamic_cast<Column*>(aspect);
 		DEBUG("row count = " << (static_cast<QVector<double>* >(column->data()))->size());
 		if (!column || column->rowCount() < 1) {
@@ -320,6 +345,7 @@
 
 	ui.lFunction->setText(text);
 	ui.teEquation->setVariables(vars);
+	checkValues();
 }
 
 void FunctionValuesDialog::generate() {