diff --git a/Modules/Filtering/DiffusionTensorImage/include/itkDiffusionTensor3DReconstructionImageFilter.h b/Modules/Filtering/DiffusionTensorImage/include/itkDiffusionTensor3DReconstructionImageFilter.h
index bb985b355e0..e222901550e 100644
--- a/Modules/Filtering/DiffusionTensorImage/include/itkDiffusionTensor3DReconstructionImageFilter.h
+++ b/Modules/Filtering/DiffusionTensorImage/include/itkDiffusionTensor3DReconstructionImageFilter.h
@@ -249,7 +249,7 @@ class ITK_TEMPLATE_EXPORT DiffusionTensor3DReconstructionImageFilter
   {
     if (idx >= m_NumberOfGradientDirections)
     {
-      itkExceptionMacro(<< "Gradient direction " << idx << "does not exist");
+      itkExceptionMacro(<< "Gradient direction " << idx << " does not exist");
     }
     return m_GradientDirectionContainer->ElementAt(idx);
   }