diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index ea4cfde33f1..1dcd8e19ed2 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -1670,6 +1670,7 @@ finalize_partialaggregate(AggState *aggstate,
 														pergroupstate->transValueIsNull,
 														pertrans->transtypeLen);
 			fcinfo->argnull[0] = pergroupstate->transValueIsNull;
+			fcinfo->isnull = false;
 
 			*resultVal = FunctionCallInvoke(fcinfo);
 			*resultIsNull = fcinfo->isnull;
diff --git a/src/include/fmgr.h b/src/include/fmgr.h
index 0216965bfc1..5c614e79fe9 100644
--- a/src/include/fmgr.h
+++ b/src/include/fmgr.h
@@ -133,6 +133,11 @@ extern void fmgr_info_copy(FmgrInfo *dstinfo, FmgrInfo *srcinfo,
  * caller must still check fcinfo->isnull!	Also, if function is strict,
  * it is caller's responsibility to verify that no null arguments are present
  * before calling.
+ *
+ * Some code performs multiple calls without redoing InitFunctionCallInfoData,
+ * possibly altering the argument values.  This is okay, but be sure to reset
+ * the fcinfo->isnull flag before each call, since callees are permitted to
+ * assume that starts out false.
  */
 #define FunctionCallInvoke(fcinfo)	((* (fcinfo)->flinfo->fn_addr) (fcinfo))