diff --git a/pda/split.cpp b/pda/split.cpp
index 05b89a228..a63b75be7 100644
--- a/pda/split.cpp
+++ b/pda/split.cpp
@@ -93,6 +93,7 @@ void Split::invert() {
 	for (iterator uit = begin(); uit != end(); uit++)
 	{
 		int num_bits = (uit+1 == end()) ? ntaxa % UINT_BITS : UINT_BITS;
+        if (num_bits == 0) num_bits = UINT_BITS; // for the case when ntaxa % UINT_BITS == 0
 
 		*uit = (1 << (num_bits-1)) - 1 + (1 << (num_bits-1)) - (*uit);
 	}