diff --git a/plugins/wimaxasncp/packet-wimaxasncp.c b/plugins/wimaxasncp/packet-wimaxasncp.c
index 4fc7dc0e93..40b55c64b4 100644
--- a/plugins/wimaxasncp/packet-wimaxasncp.c
+++ b/plugins/wimaxasncp/packet-wimaxasncp.c
@@ -2196,7 +2196,7 @@ static char *alnumerize(
     
     for ( ; (c = *r); ++r) 
     {
-        if (isalnum(c) || c == '_' || c == '.') 
+        if (isalnum((unsigned char)c) || c == '_' || c == '.') 
         {
             *(w++) = c;
         }