diff --git a/src/Microsoft.ML.Data/Transforms/ColumnConcatenatingEstimator.cs b/src/Microsoft.ML.Data/Transforms/ColumnConcatenatingEstimator.cs
index c78a5e38c3..8d7aa80763 100644
--- a/src/Microsoft.ML.Data/Transforms/ColumnConcatenatingEstimator.cs
+++ b/src/Microsoft.ML.Data/Transforms/ColumnConcatenatingEstimator.cs
@@ -87,12 +87,12 @@ private SchemaShape.Column CheckInputsAndMakeColumn(
                 // Appending keys makes no real sense anyway.
                 if (col.IsKey)
                 {
-                    throw _host.Except($"Column '{sources[i]}' is key." +
+                    throw _host.Except($"Column '{sources[i]}' is key. " +
                         $"Concatenation of keys is unsupported.");
                 }
                 if (!col.ItemType.Equals(itemType))
                 {
-                    throw _host.Except($"Column '{sources[i]}' has values of {col.ItemType}" +
+                    throw _host.Except($"Column '{sources[i]}' has values of {col.ItemType}, " +
                         $"which is not the same as earlier observed type of {itemType}.");
                 }
                 varVector |= col.Kind == SchemaShape.Column.VectorKind.VariableVector;