diff --git a/pipeline.py b/pipeline.py
index a38029d..c1c5719 100644
--- a/pipeline.py
+++ b/pipeline.py
@@ -253,7 +253,7 @@ class Components :
 							# 			skip_columns.append(_name)
 				if x_cols :
 					for _col in x_cols :
-						_df[_col] = self.approximate(df[_col])
+						_df[_col] = self.approximate(df[_col].fillna(-1))
 				#
 				# We perform a series of set operations to insure that the following conditions are met:
 				#	- the synthetic dataset only has fields that need to be synthesized