diff --git a/cfg/windows.cfg b/cfg/windows.cfg
index c1a33e30b..41c8859c7 100644
--- a/cfg/windows.cfg
+++ b/cfg/windows.cfg
@@ -5866,7 +5866,7 @@ HFONT CreateFont(
   <!-- int _tolower(int c); -->
   <function name="_tolower">
     <use-retval/>
-    <returnValue type="int"/>
+    <returnValue type="int">arg1 &lt; 'A' || arg1 &gt; 'Z' ? arg1 : arg1 + 32</returnValue>
     <noreturn>false</noreturn>
     <leak-ignore/>
     <arg nr="1" direction="in">
@@ -5904,7 +5904,7 @@ HFONT CreateFont(
   <!-- int _toupper(int c); -->
   <function name="_toupper">
     <use-retval/>
-    <returnValue type="int"/>
+    <returnValue type="int">arg1 &lt; 'a' || arg1 &gt; 'z' ? arg1 : arg1 - 32</returnValue>
     <noreturn>false</noreturn>
     <leak-ignore/>
     <arg nr="1" direction="in">