diff --git a/ProcessLib/BoundaryCondition/DirichletBoundaryConditionAuxiliaryFunctions.cpp b/ProcessLib/BoundaryCondition/DirichletBoundaryConditionAuxiliaryFunctions.cpp
index b22f8da30d5396ac02c72eb9cb405a82f7705474..471b4b9f462a8f705723a6a504a62b0da963e9b1 100644
--- a/ProcessLib/BoundaryCondition/DirichletBoundaryConditionAuxiliaryFunctions.cpp
+++ b/ProcessLib/BoundaryCondition/DirichletBoundaryConditionAuxiliaryFunctions.cpp
@@ -44,8 +44,10 @@ void checkParametersOfDirichletBoundaryCondition(
     {
         OGS_FATAL(
             "The required bulk node ids map does not exist in the boundary "
-            "mesh '{:s}'.",
-            bc_mesh.getName());
+            "mesh '{:s}' or has the wrong data type (should be equivalent to "
+            "C++ data type std::size_t which is an unsigned integer of size "
+            "{:d} or UInt64 in vtk terminology).",
+            bc_mesh.getName(), sizeof(std::size_t));
     }
 
     DBUG(