diff --git a/src/query.c b/src/query.c
index d5bd922..4284184 100644
--- a/src/query.c
+++ b/src/query.c
@@ -514,7 +514,7 @@ void extractClauseFromBooleanTest(Relids base_relids, BooleanTest *node, List **
 				break;
 			case IS_NOT_FALSE:
 				opname = "IS NOT";
-				val = (Expr *) makeConst(BOOLOID, -1, InvalidOid, sizeof(bool), BoolGetDatum(true), false, true);
+				val = (Expr *) makeConst(BOOLOID, -1, InvalidOid, sizeof(bool), BoolGetDatum(false), false, true);
 				break;
 			case IS_UNKNOWN:
 				opname = "IS";
diff --git a/test-3.9/expected/multicorn_regression_test.out b/test-3.9/expected/multicorn_regression_test.out
index f60a99a..67ec0cd 100644
--- a/test-3.9/expected/multicorn_regression_test.out
+++ b/test-3.9/expected/multicorn_regression_test.out
@@ -519,6 +519,20 @@ NOTICE:  ['test1', 'test2', 'test3']
 -------+-------+-------
 (0 rows)
 
+select * from testmulticorn where test1 IS NOT TRUE;
+NOTICE:  [test1 IS NOT t]
+NOTICE:  ['test1', 'test2', 'test3']
+ test1 | test2 | test3 
+-------+-------+-------
+(0 rows)
+
+select * from testmulticorn where test1 IS NOT FALSE;
+NOTICE:  [test1 IS NOT f]
+NOTICE:  ['test1', 'test2', 'test3']
+ test1 | test2 | test3 
+-------+-------+-------
+(0 rows)
+
 select * from testmulticorn where test1 IS UNKNOWN;
 NOTICE:  [test1 IS None]
 NOTICE:  ['test1', 'test2', 'test3']
diff --git a/test-3.9/sql/multicorn_regression_test.sql b/test-3.9/sql/multicorn_regression_test.sql
index 18d3f5b..e7bfb81 100644
--- a/test-3.9/sql/multicorn_regression_test.sql
+++ b/test-3.9/sql/multicorn_regression_test.sql
@@ -115,6 +115,8 @@ select * from testmulticorn where test1 = TRUE;
 select * from testmulticorn where test1 = FALSE;
 select * from testmulticorn where test1 IS TRUE;
 select * from testmulticorn where test1 IS FALSE;
+select * from testmulticorn where test1 IS NOT TRUE;
+select * from testmulticorn where test1 IS NOT FALSE;
 select * from testmulticorn where test1 IS UNKNOWN;
 select * from testmulticorn where test1 IS NOT UNKNOWN;