diff --git a/base/system/msiexec/msiexec.c b/base/system/msiexec/msiexec.c
index 2ed055771f6..5ebfea78ebf 100644
--- a/base/system/msiexec/msiexec.c
+++ b/base/system/msiexec/msiexec.c
@@ -393,7 +393,7 @@ static DWORD DoUnregServer(void)
     return ret;
 }
 
-extern UINT __wine_msi_call_dll_function(GUID *guid);
+extern UINT CDECL __wine_msi_call_dll_function(GUID *guid);
 
 static int DoEmbedding(LPCWSTR key)
 {
diff --git a/dll/win32/msi/custom.c b/dll/win32/msi/custom.c
index 6ffc27aa4ba..d444960c96f 100644
--- a/dll/win32/msi/custom.c
+++ b/dll/win32/msi/custom.c
@@ -485,7 +485,7 @@ static void handle_msi_break(LPCSTR target)
 static WCHAR ncalrpcW[] = {'n','c','a','l','r','p','c',0};
 static WCHAR endpoint_lrpcW[] = {'m','s','i',0};
 
-UINT __wine_msi_call_dll_function(const GUID *guid)
+UINT CDECL __wine_msi_call_dll_function(const GUID *guid)
 {
     MsiCustomActionEntryPoint fn;
     MSIHANDLE remote_package = 0;