diff --git a/src/common/string.cpp b/src/common/string.cpp
index 43ee90df20..d1cf41a7c4 100644
--- a/src/common/string.cpp
+++ b/src/common/string.cpp
@@ -1407,12 +1407,14 @@ const wxCharBuffer wxString::ToAscii() const
     // this will allocate enough space for the terminating NUL too
     wxCharBuffer buffer(length());
 
-    signed char *dest = (signed char *)buffer.data();
+    #define LOCAL_DEST_TYPE signed char
+
+    LOCAL_DEST_TYPE *dest = (LOCAL_DEST_TYPE *)buffer.data();
 
     const wchar_t *pwc = c_str();
     for ( ;; )
     {
-        *dest++ = *pwc > SCHAR_MAX ? wxT('_') : *pwc;
+        *dest++ = (LOCAL_DEST_TYPE)(*pwc > SCHAR_MAX ? wxT('_') : *pwc);
 
         // the output string can't have embedded NULs anyhow, so we can safely
         // stop at first of them even if we do have any
@@ -1420,6 +1422,8 @@ const wxCharBuffer wxString::ToAscii() const
             break;
     }
 
+    #undef LOCAL_DEST_TYPE
+
     return buffer;
 }