diff --git a/dlls/ntdll/ntdll_misc.h b/dlls/ntdll/ntdll_misc.h
index e188907283c..ca04583db53 100644
--- a/dlls/ntdll/ntdll_misc.h
+++ b/dlls/ntdll/ntdll_misc.h
@@ -258,5 +258,6 @@ NTSTATUS WINAPI RtlHashUnicodeString(PCUNICODE_STRING,BOOLEAN,ULONG,ULONG*);
 void     WINAPI LdrInitializeThunk(CONTEXT*,void**,ULONG_PTR,ULONG_PTR);
 
 /* string functions */
+int __cdecl NTDLL_tolower( int c );
 int __cdecl _stricmp( LPCSTR str1, LPCSTR str2 );
 #endif
diff --git a/dlls/ntdll/printf.c b/dlls/ntdll/printf.c
index 8a966304633..c1a019462f8 100644
--- a/dlls/ntdll/printf.c
+++ b/dlls/ntdll/printf.c
@@ -378,7 +378,7 @@ static void pf_fixup_exponent( char *buf )
 {
     char* tmp = buf;
 
-    while (tmp[0] && toupper(tmp[0]) != 'E')
+    while (tmp[0] && NTDLL_tolower(tmp[0]) != 'e')
         tmp++;
 
     if (tmp[0] && (tmp[1] == '+' || tmp[1] == '-') &&
@@ -637,7 +637,7 @@ static int pf_vsnprintf( pf_output *out, const WCHAR *format, __ms_va_list valis
             if( pf_is_double_format( flags.Format ) )
             {
                 sprintf( x, fmt, va_arg(valist, double) );
-                if (toupper(flags.Format) == 'E' || toupper(flags.Format) == 'G')
+                if (NTDLL_tolower(flags.Format) == 'e' || NTDLL_tolower(flags.Format) == 'g')
                     pf_fixup_exponent( x );
             }
             else