diff --git a/src/test/modules/test_extensions/Makefile b/src/test/modules/test_extensions/Makefile
index 7428f15ae3..07c26dd055 100644
--- a/src/test/modules/test_extensions/Makefile
+++ b/src/test/modules/test_extensions/Makefile
@@ -15,6 +15,9 @@ DATA = test_ext1--1.0.sql test_ext2--1.0.sql test_ext3--1.0.sql \
 
 REGRESS = test_extensions test_extdepend
 
+# force C locale for output stability
+NO_LOCALE = 1
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)