diff --git a/DuetPkg/DxeIpl/Debug.c b/DuetPkg/DxeIpl/Debug.c
index 1cb7e39c80..800f0080e0 100644
--- a/DuetPkg/DxeIpl/Debug.c
+++ b/DuetPkg/DxeIpl/Debug.c
@@ -52,6 +52,7 @@ ClearScreen (
 }
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...
diff --git a/DuetPkg/DxeIpl/Debug.h b/DuetPkg/DxeIpl/Debug.h
index 7fcad268ee..81bb826857 100644
--- a/DuetPkg/DxeIpl/Debug.h
+++ b/DuetPkg/DxeIpl/Debug.h
@@ -27,6 +27,7 @@ PrintHeader (
   );
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...
diff --git a/DuetPkg/EfiLdr/Debug.c b/DuetPkg/EfiLdr/Debug.c
index 84b2772112..07272928cd 100644
--- a/DuetPkg/EfiLdr/Debug.c
+++ b/DuetPkg/EfiLdr/Debug.c
@@ -49,6 +49,7 @@ ClearScreen (
 }
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...
diff --git a/DuetPkg/EfiLdr/Debug.h b/DuetPkg/EfiLdr/Debug.h
index f6aa7a2cf7..4252f31d6f 100644
--- a/DuetPkg/EfiLdr/Debug.h
+++ b/DuetPkg/EfiLdr/Debug.h
@@ -27,6 +27,7 @@ PrintHeader (
   );
 
 VOID
+EFIAPI
 PrintString (
   IN CONST CHAR8  *FormatString,
   ...