diff --git a/src/winmain.c b/src/winmain.c
index fb9a81738..ed5ac150d 100644
--- a/src/winmain.c
+++ b/src/winmain.c
@@ -674,7 +674,7 @@ static int MakeArgcArgv(char *cmdline,int *argc,char ***argv)
             strtok */
         /* trim all the whitespace off the end of the string. */
         for (i=(signed)strlen(pWorkString)-1; i >=0; i--)
-            if (isspace(pWorkString[i]))
+            if (isspace((unsigned char) pWorkString[i]))
                 pWorkString[i] = '\0';
             else
                 break;
@@ -1395,7 +1395,7 @@ rlead(char *s)
    bool has_space = TRUE;
    for(i=0;s[i] != '\0';i++)
    {
-      if(isspace(s[i]) && has_space)
+      if(isspace((unsigned char) s[i]) && has_space)
       {
          ; //Do nothing
       }
diff --git a/src/xspice/cmpp/mod_yacc.y b/src/xspice/cmpp/mod_yacc.y
index cd7da266c..bf120cd92 100644
--- a/src/xspice/cmpp/mod_yacc.y
+++ b/src/xspice/cmpp/mod_yacc.y
@@ -138,7 +138,7 @@ local_strcmpi(char *s, char *t)
    if (! (*s || *t)) {
       return 0;
    }
-   return (tolower(*s) - tolower(*t));
+   return tolower((unsigned char) *s) - tolower((unsigned char) *t);
 }
 
 /*---------------------------------------------------------------------------*/