diff --git a/c/mapping/lrec_evaluators.c b/c/mapping/lrec_evaluators.c
index 4604a2c45..f197395b1 100644
--- a/c/mapping/lrec_evaluators.c
+++ b/c/mapping/lrec_evaluators.c
@@ -110,8 +110,7 @@ mv_t lrec_evaluator_f_f_func(lrec_t* prec, context_t* pctx, void* pvstate) {
 
 	NULL_OR_ERROR_OUT(val1);
 	mt_get_double_nullable(&val1);
-	if (val1.type == MT_NULL)
-		return val1;
+	NULL_OUT(val1);
 	if (val1.type != MT_DOUBLE)
 		return MV_ERROR;
 
@@ -142,16 +141,14 @@ mv_t lrec_evaluator_f_ff_func(lrec_t* prec, context_t* pctx, void* pvstate) {
 	mv_t val1 = pstate->parg1->pevaluator_func(prec, pctx, pstate->parg1->pvstate);
 	NULL_OR_ERROR_OUT(val1);
 	mt_get_double_nullable(&val1);
-	if (val1.type == MT_NULL)
-		return val1;
+	NULL_OUT(val1);
 	if (val1.type != MT_DOUBLE)
 		return MV_ERROR;
 
 	mv_t val2 = pstate->parg2->pevaluator_func(prec, pctx, pstate->parg2->pvstate);
 	NULL_OR_ERROR_OUT(val2);
 	mt_get_double_nullable(&val2);
-	if (val2.type == MT_NULL)
-		return val2;
+	NULL_OUT(val2);
 	if (val2.type != MT_DOUBLE)
 		return MV_ERROR;
 
diff --git a/c/mapping/mlr_val.h b/c/mapping/mlr_val.h
index c646d1239..be31ec0dd 100644
--- a/c/mapping/mlr_val.h
+++ b/c/mapping/mlr_val.h
@@ -44,6 +44,10 @@ extern mv_t MV_ERROR;
 		return MV_NULL; \
 }
 
+#define NULL_OUT(val) { \
+	if ((val).type == MT_NULL) \
+		return MV_NULL; \
+}
 #define ERROR_OUT(val) { \
 	if ((val).type == MT_ERROR) \
 		return MV_ERROR; \
diff --git a/c/todo.txt b/c/todo.txt
index e008aa6d8..84398ca54 100644
--- a/c/todo.txt
+++ b/c/todo.txt
@@ -7,7 +7,7 @@ TOP OF LIST
 * make a -D for hash-collision stats ...
 
 * go through remaining functions to decide when null-through is ok.
-* also document thoroughly.
+* also document thoroughly. emphasize this is crucial for heterogeneous data.
 
 * doc w/ very specific examples of sed/grep/etc preprocessing to structurize semi-structured data (e.g. logs)