diff --git a/pkgs/by-name/_9/_9base/package.nix b/pkgs/by-name/_9/_9base/package.nix
index 809b9da39af4..6f53870c36cf 100644
--- a/pkgs/by-name/_9/_9base/package.nix
+++ b/pkgs/by-name/_9/_9base/package.nix
@@ -47,6 +47,9 @@ stdenv.mkDerivation {
     # hide really common warning that floods the logs:
     #   warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
     "-D_DEFAULT_SOURCE"
+  ] ++ lib.optionals stdenv.cc.isClang [
+    # error: call to undeclared function 'p9mbtowc'; ISO C99 and later do not support implicit function declarations
+    "-Wno-error=implicit-function-declaration"
   ];
   LDFLAGS = lib.optionalString enableStatic "-static";
   makeFlags = [