diff --git a/common/configuration.cpp b/common/configuration.cpp
index e0f21feb9b6a30a1066927c8b6ea521cc27fbc26..11b17ee3e6c6d52ab2e8ad0c6ffb8840d0c80328 100644
--- a/common/configuration.cpp
+++ b/common/configuration.cpp
@@ -53,7 +53,7 @@ bool ConfigurationInterface::pntIsValid(unsigned int e_r) const
 {
   if( !(e_r<_np) )
   {
-    throw Exception("Emitter-receiver "+std::to_string(e_r)+" is out of scope.");
+    throw Exception("Emitter-receiver "+std::to_string(e_r)+" is out of scope. There are only "+std::to_string(_np)+" points.");
   }
   return true;
 }
diff --git a/specific/configuration/flexible_acquisition.cpp b/specific/configuration/flexible_acquisition.cpp
index 81b9e17f0d8699f06ea0d1f389bb0f7d2cbdec1c..c6adf1de3a33cdc02d55d9d9aca7f1935786c452 100644
--- a/specific/configuration/flexible_acquisition.cpp
+++ b/specific/configuration/flexible_acquisition.cpp
@@ -70,6 +70,9 @@ namespace flexible_acquisition
           _er_positions.push_back(coords);
         }
 
+        _np = _er_positions.size();
+
+
         mesh();
 
         
@@ -86,6 +89,7 @@ namespace flexible_acquisition
         }
 
 
+
         /*
         * DOMAIN
         */