diff --git a/dlls/api-ms-win-crt-private-l1-1-0/api-ms-win-crt-private-l1-1-0.spec b/dlls/api-ms-win-crt-private-l1-1-0/api-ms-win-crt-private-l1-1-0.spec
index 1d5e2f84c86..7e2717c44ab 100644
--- a/dlls/api-ms-win-crt-private-l1-1-0/api-ms-win-crt-private-l1-1-0.spec
+++ b/dlls/api-ms-win-crt-private-l1-1-0/api-ms-win-crt-private-l1-1-0.spec
@@ -613,7 +613,7 @@
 @ stub _o__mbstok_s_l
 @ stub _o__mbstowcs_l
 @ stub _o__mbstowcs_s_l
-@ stub _o__mbstrlen
+@ cdecl _o__mbstrlen(str) ucrtbase._o__mbstrlen
 @ stub _o__mbstrlen_l
 @ stub _o__mbstrnlen
 @ stub _o__mbstrnlen_l
diff --git a/dlls/ucrtbase/ucrtbase.spec b/dlls/ucrtbase/ucrtbase.spec
index 070f0d607c5..0d7616c45c8 100644
--- a/dlls/ucrtbase/ucrtbase.spec
+++ b/dlls/ucrtbase/ucrtbase.spec
@@ -1277,7 +1277,7 @@
 @ stub _o__mbstok_s_l
 @ stub _o__mbstowcs_l
 @ stub _o__mbstowcs_s_l
-@ stub _o__mbstrlen
+@ cdecl _o__mbstrlen(str) _mbstrlen
 @ stub _o__mbstrlen_l
 @ stub _o__mbstrnlen
 @ stub _o__mbstrnlen_l