diff --git a/src/ModelingAlgorithms/TKFillet/ChFi3d/ChFi3d_Builder_2.cxx b/src/ModelingAlgorithms/TKFillet/ChFi3d/ChFi3d_Builder_2.cxx
index 2caef49094..9a6c5801a5 100644
--- a/src/ModelingAlgorithms/TKFillet/ChFi3d/ChFi3d_Builder_2.cxx
+++ b/src/ModelingAlgorithms/TKFillet/ChFi3d/ChFi3d_Builder_2.cxx
@@ -79,6 +79,11 @@ namespace
 static inline occ::handle<Geom2d_Curve> getCurveOnSurface(const TopoDS_Edge& theEdge,
                                                           const TopoDS_Face& theFace)
 {
+  if (theEdge.IsNull() || theFace.IsNull())
+  {
+    return occ::handle<Geom2d_Curve>();
+  }
+
   double aFirstParam = 0.;
   double aLastParam  = 0.;
   return BRep_Tool::CurveOnSurface(theEdge, theFace, aFirstParam, aLastParam);
@@ -1189,6 +1194,10 @@ bool ChFi3d_Builder::StartSol(
   const TopOpeBRepDS_DataStructure& DStr = myDS->ChangeDS();
   TopoDS_Face                       Fref = !HSref.IsNull() ? HSref->Face() : TopoDS_Face();
   TopoDS_Face                       F    = TopoDS::Face(DStr.Shape(SD->Index(ons)));
+  if (F.IsNull())
+  {
+    return false;
+  }
 
   const ChFiDS_CommonPoint& aCommonPoint = SD->Vertex(isfirst, ons);
   HSBis.Nullify();
@@ -1209,8 +1218,12 @@ bool ChFi3d_Builder::StartSol(
       if (isExtend && !aCommonPoint.Point().IsEqual(CPbis.Point(), 0))
       {
         //  the state is preserved and False is returned (extension by the expected plane).
-        HS->Initialize(F);
         aPCurve = SD->Interference(ons).PCurveOnFace();
+        if (aPCurve.IsNull())
+        {
+          return false;
+        }
+        HS->Initialize(F);
         // The 2nd point is given by its trace on the support surface
         RecS = false;
         pons = aPCurve->Value(tns);
@@ -1225,6 +1238,10 @@ bool ChFi3d_Builder::StartSol(
     // eventually the support face and(or) the reference face.
     const TopoDS_Vertex VCP = aCommonPoint.Vertex();
     const TopoDS_Edge   EHC = HC->Edge();
+    if (VCP.IsNull() || EHC.IsNull() || Fref.IsNull() || HSref.IsNull() || HCref.IsNull())
+    {
+      return false;
+    }
     // One starts by searching in Fref another edge referencing VCP.
     TopoDS_Edge newedge;
     TopoDS_Edge edgereg;
@@ -1330,9 +1347,12 @@ bool ChFi3d_Builder::StartSol(
     }
     else
     {
-      HS->Initialize(Fv);
-      W = BRep_Tool::Parameter(VCP, newedge);
-      HCref->Initialize(newedge, Fref);
+      const occ::handle<Geom2d_Curve> aRefPCurve = getCurveOnSurface(newedge, Fref);
+      if (aRefPCurve.IsNull())
+      {
+        return false;
+      }
+
       TopoDS_Face newface = Fv;
       newface.Orientation(TopAbs_FORWARD);
       for (TopExp_Explorer ex(newface, TopAbs_EDGE); ex.More(); ex.Next())
@@ -1343,6 +1363,14 @@ bool ChFi3d_Builder::StartSol(
           break;
         }
       }
+      const occ::handle<Geom2d_Curve> aNewPCurve = getCurveOnSurface(newedge, Fv);
+      if (aNewPCurve.IsNull())
+      {
+        return false;
+      }
+      HS->Initialize(Fv);
+      W = BRep_Tool::Parameter(VCP, newedge);
+      HCref->Initialize(newedge, Fref);
       HC->Initialize(newedge, Fv);
       pons = HC->Value(W);
     }
@@ -1363,28 +1391,52 @@ bool ChFi3d_Builder::StartSol(
   if (aCommonPoint.IsOnArc())
   {
     const TopoDS_Edge& anArcEdge = aCommonPoint.Arc();
+    if (anArcEdge.IsNull())
+    {
+      return false;
+    }
 
     // Lambda to avoid code duplication.
-    // Sets HS, W, aPCurve and pons to default return values.
-    auto prepareDefaultReturn = [&]() {
-      HS->Initialize(F);
-      W       = aCommonPoint.ParameterOnArc();
+    // Attempts to set HS, W, aPCurve and pons to default return values.
+    auto prepareDefaultReturn = [&]() -> bool {
       aPCurve = getCurveOnSurface(anArcEdge, F);
-      pons    = aPCurve->Value(W);
+      if (aPCurve.IsNull())
+      {
+        return false;
+      }
+      HS->Initialize(F);
+      W    = aCommonPoint.ParameterOnArc();
+      pons = aPCurve->Value(W);
+      return true;
     };
 
     if (decroch)
     {
-      HS->Initialize(Fref);
-      W       = aCommonPoint.ParameterOnArc();
+      if (Fref.IsNull())
+      {
+        prepareDefaultReturn();
+        return false;
+      }
       aPCurve = getCurveOnSurface(anArcEdge, Fref);
-      pons    = aPCurve->Value(W);
-      RecS    = true;
+      if (aPCurve.IsNull())
+      {
+        prepareDefaultReturn();
+        return false;
+      }
+      HS->Initialize(Fref);
+      W    = aCommonPoint.ParameterOnArc();
+      pons = aPCurve->Value(W);
+      RecS = true;
       return true;
     }
 
     if (SearchFace(Spine, aCommonPoint, F, Fv))
     {
+      if (Fv.IsNull())
+      {
+        prepareDefaultReturn();
+        return false;
+      }
       HS->Initialize(Fv);
       RecS = true;
       if (aCommonPoint.IsVertex())
@@ -1420,10 +1472,20 @@ bool ChFi3d_Builder::StartSol(
         if (isTangentToArc(aCommonPoint, 0.1))
         {
           aPCurve = getCurveOnSurface(aCommonPoint.Arc(), F);
-          HSBis   = new BRepAdaptor_Surface(F);
-          PBis    = aPCurve->Value(aCommonPoint.ParameterOnArc());
+          if (aPCurve.IsNull())
+          {
+            prepareDefaultReturn();
+            return false;
+          }
+          HSBis = new BRepAdaptor_Surface(F);
+          PBis  = aPCurve->Value(aCommonPoint.ParameterOnArc());
         }
 
+        if (newedge.IsNull())
+        {
+          prepareDefaultReturn();
+          return false;
+        }
         aPCurve = getCurveOnSurface(newedge, Fv);
       }
       else
@@ -1433,6 +1495,11 @@ bool ChFi3d_Builder::StartSol(
         Fv.Orientation(TopAbs_FORWARD);
         aPCurve = getCurveOnSurface(newedge, Fv);
       }
+      if (aPCurve.IsNull())
+      {
+        prepareDefaultReturn();
+        return false;
+      }
       pons = aPCurve->Value(aCommonPoint.ParameterOnArc());
       return true;
     }
@@ -1452,6 +1519,11 @@ bool ChFi3d_Builder::StartSol(
       }
       if (c1obstacle)
       {
+        if (HSref.IsNull() || HCref.IsNull())
+        {
+          prepareDefaultReturn();
+          return false;
+        }
         HS->Initialize(Fv);
         HSref->Initialize(F);
         W  = aCommonPoint.ParameterOnArc();