diff --git a/src/Mod/Sketcher/App/Sketch.cpp b/src/Mod/Sketcher/App/Sketch.cpp
index 0d1ca669a..36839636e 100644
--- a/src/Mod/Sketcher/App/Sketch.cpp
+++ b/src/Mod/Sketcher/App/Sketch.cpp
@@ -1868,6 +1868,9 @@ int Sketch::setDatum(int constrId, double value)
 
 int Sketch::getPointId(int geoId, PointPos pos) const
 {
+    // do a range check first
+    if (geoId < 0 || geoId >= Geoms.size())
+        return -1;
     switch (pos) {
     case start:
         return Geoms[geoId].startPointId;