diff --git a/src/tools/valgrind.supp b/src/tools/valgrind.supp
index ec47a228ae..8512231539 100644
--- a/src/tools/valgrind.supp
+++ b/src/tools/valgrind.supp
@@ -212,3 +212,39 @@
    Memcheck:Cond
    fun:PyObject_Realloc
 }
+
+# wcsrtombs uses some clever optimizations internally, which to valgrind
+# may look like access to uninitialized data. For example AVX2 instructions
+# load data in 256-bit chunks, irrespectedly of wchar length. gconv does
+# somethink similar by loading data in 32bit chunks and then shifting the
+# data internally. Neither of those actually uses the uninitialized part
+# of the buffer, as far as we know.
+#
+# https://www.postgresql.org/message-id/90ac0452-e907-e7a4-b3c8-15bd33780e62@2ndquadrant.com
+
+{
+   wcsnlen_optimized
+   Memcheck:Cond
+   ...
+   fun:wcsrtombs
+   fun:wcstombs
+   fun:wchar2char
+}
+
+{
+   wcsnlen_optimized_addr32
+   Memcheck:Addr32
+   ...
+   fun:wcsrtombs
+   fun:wcstombs
+   fun:wchar2char
+}
+
+{
+   gconv_transform_internal
+   Memcheck:Cond
+   fun:__gconv_transform_internal_utf8
+   fun:wcsrtombs
+   fun:wcstombs
+   fun:wchar2char
+}