diff --git a/include/wx/generic/gridsel.h b/include/wx/generic/gridsel.h
index cb53078f2c..1a64387c84 100644
--- a/include/wx/generic/gridsel.h
+++ b/include/wx/generic/gridsel.h
@@ -73,9 +73,10 @@ public:
     // wxGrid::m_currentCellCoords (the exception is when we scrolled out from
     // the top of the grid and select a column or scrolled right and select
     // a row: in this case the lowest visible row/column will be set as
-    // current, not the first one). If the row or the column component of
-    // blockEnd parameter has value of -1, it means that the corresponding
-    // component of the current block should not be changed.
+    // current, not the first one).
+    //
+    // Both components of both blockStart and blockEnd must be valid.
+    //
     // Return true if the current block was actually changed or created.
     bool ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockStart,
                                     const wxGridCellCoords& blockEnd,
diff --git a/src/generic/grid.cpp b/src/generic/grid.cpp
index 56f1cfec6a..e8b0734dfa 100644
--- a/src/generic/grid.cpp
+++ b/src/generic/grid.cpp
@@ -7925,12 +7925,17 @@ bool
 wxGrid::PrepareForSelectionExpansion(wxGridCellCoords& coords,
                                      const wxGridDirectionOperations& diroper)
 {
-    coords.SetRow(m_selection->GetCurrentBlockCornerRow());
-    coords.SetCol(m_selection->GetCurrentBlockCornerCol());
+    int row = m_selection->GetCurrentBlockCornerRow();
+    if ( row == -1 )
+        row = m_currentCellCoords.GetRow();
 
-    if ( coords == wxGridNoCellCoords )
-        coords = m_currentCellCoords;
-    else if ( !diroper.IsValid(coords) )
+    int col = m_selection->GetCurrentBlockCornerCol();
+    if ( col == -1 )
+        col = m_currentCellCoords.GetCol();
+
+    coords.Set(row, col);
+
+    if ( !diroper.IsValid(coords) )
     {
         // The component of the current block corner in our direction
         // is not valid. This means we can't change the selection block
diff --git a/src/generic/gridsel.cpp b/src/generic/gridsel.cpp
index cc361a0d97..963717aac0 100644
--- a/src/generic/gridsel.cpp
+++ b/src/generic/gridsel.cpp
@@ -486,6 +486,9 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
                                                  const wxGridCellCoords& blockEnd,
                                                  const wxKeyboardState& kbd)
 {
+    wxASSERT( blockStart.GetRow() != -1 && blockStart.GetCol() != -1 &&
+              blockEnd.GetRow() != -1 && blockEnd.GetCol() != -1 );
+
     if ( m_selection.empty() )
     {
         SelectBlock(blockStart, blockEnd);
@@ -497,42 +500,34 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
 
     bool editBlock = false;
 
-    if ( blockEnd.GetRow() != -1 )
+    // If the new block starts at the same top row as the current one, the
+    // end block coordinates must correspond to the new bottom row -- and
+    // vice versa, if the new block starts at the bottom, its other end
+    // must correspond to the top.
+    if ( blockStart.GetRow() == block.GetTopRow() )
     {
-        // If the new block starts at the same top row as the current one, the
-        // end block coordinates must correspond to the new bottom row -- and
-        // vice versa, if the new block starts at the bottom, its other end
-        // must correspond to the top.
-        if ( blockStart.GetRow() == block.GetTopRow() )
-        {
-            newBlock.SetBottomRow(blockEnd.GetRow());
-            editBlock = true;
-        }
-        else if ( blockStart.GetRow() == block.GetBottomRow() )
-        {
-            newBlock.SetTopRow(blockEnd.GetRow());
-            editBlock = true;
-        }
+        newBlock.SetBottomRow(blockEnd.GetRow());
+        editBlock = true;
     }
-    if ( blockEnd.GetCol() != -1 )
+    else if ( blockStart.GetRow() == block.GetBottomRow() )
     {
-        if ( newBlock.GetLeftCol() == blockStart.GetCol() )
-        {
-            newBlock.SetRightCol(blockEnd.GetCol());
-            editBlock = true;
-        }
-        else if ( newBlock.GetRightCol() == blockStart.GetCol() )
-        {
-            newBlock.SetLeftCol(blockEnd.GetCol());
-            editBlock = true;
-        }
+        newBlock.SetTopRow(blockEnd.GetRow());
+        editBlock = true;
+    }
+
+    if ( blockStart.GetCol() == block.GetLeftCol() )
+    {
+        newBlock.SetRightCol(blockEnd.GetCol());
+        editBlock = true;
+    }
+    else if ( blockStart.GetCol() == block.GetRightCol() )
+    {
+        newBlock.SetLeftCol(blockEnd.GetCol());
+        editBlock = true;
     }
 
     newBlock = newBlock.Canonicalize();
 
-    const bool endCoordsSet =
-        blockEnd.GetRow() != -1 && blockEnd.GetCol() != -1;
-
     if ( editBlock )
     {
         if ( newBlock == block )
@@ -562,15 +557,14 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
                                         true,
                                         kbd);
         m_grid->GetEventHandler()->ProcessEvent(gridEvt);
-        return true;
     }
-    else if ( endCoordsSet )
+    else
     {
         // Select the new one.
         SelectBlock(newBlock.GetTopLeft(), newBlock.GetBottomRight(), kbd);
-        return true;
     }
-    return false;
+
+    return true;
 }
 
 int wxGridSelection::GetCurrentBlockCornerRow() const