diff --git a/src/BRepFill/BRepFill_Filling.cdl b/src/BRepFill/BRepFill_Filling.cdl
index 551a9c5de6..cf4443c661 100644
--- a/src/BRepFill/BRepFill_Filling.cdl
+++ b/src/BRepFill/BRepFill_Filling.cdl
@@ -125,6 +125,11 @@ is
 
     LoadInitSurface( me : in out; aFace : Face from TopoDS );
     ---Purpose: Loads the initial Surface
+    --          The initial surface must have orthogonal local coordinates,
+    --          i.e. partial derivatives dS/du and dS/dv must be orthogonal
+    --          at each point of surface.
+    --          If this condition breaks, distortions of resulting surface
+    --          are possible.
     
 
     Add( me : in out; anEdge : Edge from TopoDS;
diff --git a/src/BRepOffsetAPI/BRepOffsetAPI_MakeFilling.cdl b/src/BRepOffsetAPI/BRepOffsetAPI_MakeFilling.cdl
index 84b6a1933a..bf6facd91d 100644
--- a/src/BRepOffsetAPI/BRepOffsetAPI_MakeFilling.cdl
+++ b/src/BRepOffsetAPI/BRepOffsetAPI_MakeFilling.cdl
@@ -146,6 +146,11 @@ is
     	-- If no initial surface is given, the algorithm computes it
     	-- automatically. If the set of edges is not connected (Free constraint),
     	-- missing edges are automatically computed.
+    	-- Important: the initial surface must have orthogonal local coordinates,
+    	-- i.e. partial derivatives dS/du and dS/dv must be orthogonal
+    	-- at each point of surface.
+    	-- If this condition breaks, distortions of resulting surface
+    	-- are possible.
     
     
     Add( me : in out; Constr : Edge from TopoDS;