diff --git a/src/plugins/tables/kexitabledesignerview.cpp b/src/plugins/tables/kexitabledesignerview.cpp
--- a/src/plugins/tables/kexitabledesignerview.cpp
+++ b/src/plugins/tables/kexitabledesignerview.cpp
@@ -392,7 +392,8 @@
                                                      xi18n("Default Value"), QString(),
 //! @todo use "Variant" type here when supported by KProperty
                                                 (KProperty::Type)field.variantType()));
-    prop->setOption("3rdState", xi18n("None"));
+    prop->setOption("3rdState", xi18n("None")); // for booleans
+    prop->setOption("nullValueAllowed", true); // since 3.2
 
     set->addProperty(prop = new KProperty("primaryKey", QVariant(field.isPrimaryKey()),
                                                      xi18n("Primary Key")));
@@ -980,13 +981,15 @@
         const KDbField::Type newType = KDbField::typeForString(property.value().toString());
         set["type"].setValue(newType);
 
-        // cast "defaultValue" property value to a new type
-        QVariant oldDefVal(set["defaultValue"].value());
-        QVariant newDefVal(tryCastQVariant(oldDefVal, KDbField::variantType(type)));
-        if (oldDefVal.type() != newDefVal.type())
-            set["defaultValue"].setType(newDefVal.type());
-        d->setPropertyValueIfNeeded(set, "defaultValue", newDefVal, newDefVal,
-                                    changeFieldTypeCommand);
+        // cast "defaultValue" property value to a new type if needed
+        const QVariant::Type newVariantType = KDbField::variantType(type);
+        set["defaultValue"].setType(static_cast<KProperty::Type>(newVariantType));
+        const QVariant oldDefaultValue(set["defaultValue"].value());
+        if (!oldDefaultValue.isNull() && newVariantType != QVariant::Invalid) { // convert value if needed
+            const QVariant newDefaultValue(tryCastQVariant(oldDefaultValue, newVariantType));
+            d->setPropertyValueIfNeeded(set, "defaultValue", newDefaultValue, oldDefaultValue,
+                                        changeFieldTypeCommand);
+        }
 
         d->updatePropertiesVisibility(newType, set);
         //properties' visiblility changed: refresh prop. set