diff --git a/python/flatbuffers/builder.py b/python/flatbuffers/builder.py
index 3f0cf4e..9bdf116 100644
--- a/python/flatbuffers/builder.py
+++ b/python/flatbuffers/builder.py
@@ -23,6 +23,8 @@
 from .compat import memoryview_type
 from .compat import import_numpy, NumpyRequiredForThisFeature
 
+import warnings
+
 np = import_numpy()
 ## @file
 ## @addtogroup flatbuffers_python_api
@@ -75,6 +77,13 @@
     """
     pass
 
+class EndVectorLengthMismatched(RuntimeError):
+    """
+    The number of elements passed to EndVector does not match the number 
+    specified in StartVector.
+    """
+    pass
+
 
 # VtableMetadataFields is the count of metadata fields in each vtable.
 VtableMetadataFields = 2
@@ -377,13 +386,20 @@
         return self.Offset()
     ## @endcond
 
-    def EndVector(self):
+    def EndVector(self, numElems = None):
         """EndVector writes data necessary to finish vector construction."""
 
         self.assertNested()
         ## @cond FLATBUFFERS_INTERNAL
         self.nested = False
         ## @endcond
+               
+        if numElems:
+            warnings.warn("numElems is deprecated.", 
+                          DeprecationWarning, stacklevel=2)
+            if numElems != self.vectorNumElems:
+                raise EndVectorLengthMismatched();
+
         # we already made space for this, so write without PrependUint32
         self.PlaceUOffsetT(self.vectorNumElems)
         self.vectorNumElems = None
@@ -562,9 +578,11 @@
         self.Place(off, flags)
 
     def PrependSlot(self, flags, o, x, d):
-        N.enforce_number(x, flags)
-        N.enforce_number(d, flags)
-        if x != d or self.forceDefaults:
+        if x is not None:
+            N.enforce_number(x, flags)
+        if d is not None:
+            N.enforce_number(d, flags)
+        if x != d or (self.forceDefaults and d is not None):
             self.Prepend(flags, x)
             self.Slot(o)