diff --git a/DuetPkg/FSVariable/FSVariable.c b/DuetPkg/FSVariable/FSVariable.c
index 70da3907b8..9a50cca72b 100644
--- a/DuetPkg/FSVariable/FSVariable.c
+++ b/DuetPkg/FSVariable/FSVariable.c
@@ -753,6 +753,7 @@ GetLangFromSupportedLangCodes (
 
 **/
 CHAR8 *
+EFIAPI
 VariableGetBestLanguage (
   IN CONST CHAR8  *SupportedLanguages, 
   IN BOOLEAN      Iso639Language,
diff --git a/MdeModulePkg/Universal/Variable/EmuRuntimeDxe/EmuVariable.c b/MdeModulePkg/Universal/Variable/EmuRuntimeDxe/EmuVariable.c
index e0cd873f96..cd0023671e 100644
--- a/MdeModulePkg/Universal/Variable/EmuRuntimeDxe/EmuVariable.c
+++ b/MdeModulePkg/Universal/Variable/EmuRuntimeDxe/EmuVariable.c
@@ -572,6 +572,7 @@ GetLangFromSupportedLangCodes (
 
 **/
 CHAR8 *
+EFIAPI
 VariableGetBestLanguage (
   IN CONST CHAR8  *SupportedLanguages, 
   IN BOOLEAN      Iso639Language,
diff --git a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
index 8123338112..dca3e30370 100644
--- a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
+++ b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
@@ -1248,6 +1248,7 @@ GetLangFromSupportedLangCodes (
 
 **/
 CHAR8 *
+EFIAPI
 VariableGetBestLanguage (
   IN CONST CHAR8  *SupportedLanguages, 
   IN BOOLEAN      Iso639Language,