diff --git a/Modules/Filtering/ImageIntensity/include/itkNaryFunctorImageFilter.hxx b/Modules/Filtering/ImageIntensity/include/itkNaryFunctorImageFilter.hxx
index e917de5107d..82b0faea7cb 100644
--- a/Modules/Filtering/ImageIntensity/include/itkNaryFunctorImageFilter.hxx
+++ b/Modules/Filtering/ImageIntensity/include/itkNaryFunctorImageFilter.hxx
@@ -30,7 +30,7 @@ NaryFunctorImageFilter< TInputImage, TOutputImage, TFunction >
 {
   // This number will be incremented each time an image
   // is added over the two minimum required
-  this->SetNumberOfRequiredInputs(2);
+  this->SetNumberOfRequiredInputs(1);
   this->InPlaceOff();
   this->DynamicMultiThreadingOn();
 }