diff --git a/frappy/gui/modulewidget.py b/frappy/gui/modulewidget.py
index 52f4b91..cbaacba 100644
--- a/frappy/gui/modulewidget.py
+++ b/frappy/gui/modulewidget.py
@@ -24,9 +24,9 @@ from frappy.gui.qt import QColor, QDialog, QHBoxLayout, QIcon, QLabel, \
     QLineEdit, QMessageBox, QPropertyAnimation, QPushButton, Qt, QToolButton, \
     QWidget, pyqtProperty, pyqtSignal
 
+from frappy.gui.inputwidgets import get_input_widget
 from frappy.gui.util import Colors, loadUi
 from frappy.gui.valuewidgets import get_widget
-from frappy.gui.inputwidgets import get_input_widget
 
 
 class CommandDialog(QDialog):
@@ -54,7 +54,11 @@ class CommandDialog(QDialog):
         self.resize(self.sizeHint())
 
     def get_value(self):
-        return True, self.widgets[0].get_value()
+        try:
+            return self.widgets[0].get_value()
+        except Exception as e:
+            QMessageBox.warning(self.parent(), 'Operation failed', str(e))
+            return None
 
     def exec(self):
         if super().exec():
@@ -95,8 +99,9 @@ class CommandButton(QPushButton):
         if self._argintype:
             dlg = CommandDialog(self._cmdname, self._argintype)
             args = dlg.exec()
-            if args:  # not 'Cancel' clicked
-                self._cb(self._cmdname, args[1])
+            if args is not None:
+                # no errors when converting value and 'Cancel' wasn't clicked
+                self._cb(self._cmdname, args)
         else:
             # no need for arguments
             self._cb(self._cmdname, None)
@@ -442,8 +447,8 @@ class ModuleWidget(QWidget):
         self.paramDetails.emit(self._name, param)
 
     def _button_pressed(self, param):
-        target = self._paramInputs[param].get_input()
         try:
+            target = self._paramInputs[param].get_input()
             self._node.setParameter(self._name, param, target)
         except Exception as e:
             QMessageBox.warning(self.parent(), 'Operation failed', str(e))