It does not make sense to classify into zero classes, does it?

src/core/symbology-ng/qgsgraduatedsymbolrendererv2.cpp

@@ -690,6 +690,8 @@ QgsGraduatedSymbolRendererV2* QgsGraduatedSymbolRendererV2::createRenderer(
   QgsSymbolV2* symbol,
   QgsVectorColorRampV2* ramp )
 {
+  if ( classes < 1 )
+    return NULL;
 
   int attrNum = vlayer->fieldNameIndex( attrName );
 

src/gui/symbology-ng/qgsgraduatedsymbolrendererv2widget.cpp

@@ -176,9 +176,16 @@ void QgsGraduatedSymbolRendererV2Widget::classifyGraduated()
     mode = QgsGraduatedSymbolRendererV2::Quantile;
 
   // create and set new renderer
+  QgsGraduatedSymbolRendererV2* r = QgsGraduatedSymbolRendererV2::createRenderer(
+                                      mLayer, attrName, classes, mode, mGraduatedSymbol, ramp );
+  if ( !r )
+  {
+    QMessageBox::critical( this, tr( "Error" ), tr( "Renderer creation has failed." ) );
+    return;
+  }
+
   delete mRenderer;
-  mRenderer = QgsGraduatedSymbolRendererV2::createRenderer(
-                mLayer, attrName, classes, mode, mGraduatedSymbol, ramp );
+  mRenderer = r;
 
   populateRanges();
 }

src/ui/qgsgraduatedsymbolrendererv2widget.ui

@@ -70,6 +70,12 @@
          <verstretch>0</verstretch>
         </sizepolicy>
        </property>
+       <property name="minimum">
+        <number>1</number>
+       </property>
+       <property name="maximum">
+        <number>999</number>
+       </property>
        <property name="value">
         <number>5</number>
        </property>