diff --git a/src/common/sizer.cpp b/src/common/sizer.cpp
index 63a832d8ff..1ff4b4ec62 100644
--- a/src/common/sizer.cpp
+++ b/src/common/sizer.cpp
@@ -1830,7 +1830,10 @@ bool wxFlexGridSizer::IsColGrowable( size_t idx )
 
 void wxFlexGridSizer::AddGrowableRow( size_t idx, int proportion )
 {
-    wxCHECK_RET( idx < (size_t)GetRows(), "invalid row index" );
+    int nrows, ncols;
+    CalcRowsCols(nrows, ncols);
+    wxCHECK_RET( idx < (size_t)nrows, "invalid row index" );
+
     wxASSERT_MSG( !IsRowGrowable( idx ),
                   "AddGrowableRow() called for growable row" );
     m_growableRows.Add( idx );
@@ -1839,7 +1842,10 @@ void wxFlexGridSizer::AddGrowableRow( size_t idx, int proportion )
 
 void wxFlexGridSizer::AddGrowableCol( size_t idx, int proportion )
 {
-    wxCHECK_RET( idx < (size_t)GetCols(), "invalid column index" );
+    int nrows, ncols;
+    CalcRowsCols(nrows, ncols);
+    wxCHECK_RET( idx < (size_t)ncols, "invalid column index" );
+
     wxASSERT_MSG( !IsColGrowable( idx ),
                   "AddGrowableCol() called for growable column" );
     m_growableCols.Add( idx );