diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index b500f77d..55d868e0 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -426,7 +426,8 @@ bool sesschan_enable_x11_forwarding(
         const unsigned char *hex = authdata_hex.ptr;
         char hexbuf[3];
 
-        if (!isxdigit(hex[i]) || !isxdigit(hex[i+1])) {
+        if (!isxdigit((unsigned char)hex[i]) ||
+            !isxdigit((unsigned char)hex[i+1])) {
             strbuf_free(authdata_bin);
             return false;              /* not hex */
         }
diff --git a/utils/validate_manual_hostkey.c b/utils/validate_manual_hostkey.c
index 7c5d1b88..dd7c1aee 100644
--- a/utils/validate_manual_hostkey.c
+++ b/utils/validate_manual_hostkey.c
@@ -63,7 +63,7 @@ bool validate_manual_hostkey(char *key)
                 if (r[3*i+2] != ':')
                     goto not_fingerprint; /* sorry */
             for (i = 0; i < 16*3 - 1; i++)
-                key[i] = tolower(r[i]);
+                key[i] = tolower((unsigned char)r[i]);
             key[16*3 - 1] = '\0';
             return true;
         }
diff --git a/windows/pageant.c b/windows/pageant.c
index cb4c1186..9c7a5696 100644
--- a/windows/pageant.c
+++ b/windows/pageant.c
@@ -1835,7 +1835,7 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
             args = strchr(command, ' ');
         if (args) {
             *args++ = 0;
-            while (*args && isspace(*args)) args++;
+            while (*args && isspace((unsigned char)*args)) args++;
         }
         spawn_cmd(command, args, show);
     }
diff --git a/windows/unicode.c b/windows/unicode.c
index 26ffcb6f..412db6ea 100644
--- a/windows/unicode.c
+++ b/windows/unicode.c
@@ -1076,9 +1076,11 @@ static int check_compose_internal(int first, int second, int recurse)
     if (recurse == 0) {
         nc = check_compose_internal(second, first, 1);
         if (nc == -1)
-            nc = check_compose_internal(toupper(first), toupper(second), 1);
+            nc = check_compose_internal(toupper((unsigned char)first),
+                                        toupper((unsigned char)second), 1);
         if (nc == -1)
-            nc = check_compose_internal(toupper(second), toupper(first), 1);
+            nc = check_compose_internal(toupper((unsigned char)second),
+                                        toupper((unsigned char)first), 1);
     }
     return nc;
 }
@@ -1102,9 +1104,9 @@ int decode_codepage(const char *cp_name)
         s = cp_name;
         d = cpi->name;
         for (;;) {
-            while (*s && !isalnum(*s) && *s != ':')
+            while (*s && !isalnum((unsigned char)*s) && *s != ':')
                 s++;
-            while (*d && !isalnum(*d) && *d != ':')
+            while (*d && !isalnum((unsigned char)*d) && *d != ':')
                 d++;
             if (*s == 0) {
                 codepage = cpi->codepage;
diff --git a/windows/utils/split_into_argv.c b/windows/utils/split_into_argv.c
index 1c17f0ce..b6daada4 100644
--- a/windows/utils/split_into_argv.c
+++ b/windows/utils/split_into_argv.c
@@ -173,7 +173,7 @@ void split_into_argv(char *cmdline, bool includes_program_name,
      * First deal with the simplest of all special cases: if there
      * aren't any arguments, return 0,NULL,NULL.
      */
-    while (*cmdline && isspace(*cmdline)) cmdline++;
+    while (*cmdline && isspace((unsigned char)*cmdline)) cmdline++;
     if (!*cmdline) {
         if (argc) *argc = 0;
         if (argv) *argv = NULL;
@@ -195,7 +195,7 @@ void split_into_argv(char *cmdline, bool includes_program_name,
         bool quote;
 
         /* Skip whitespace searching for start of argument. */
-        while (*p && isspace(*p)) p++;
+        while (*p && isspace((unsigned char)*p)) p++;
         if (!*p) break;
 
         /*
@@ -240,7 +240,7 @@ void split_into_argv(char *cmdline, bool includes_program_name,
 
         /* Copy data into the argument until it's finished. */
         while (*p) {
-            if (!quote && isspace(*p))
+            if (!quote && isspace((unsigned char)*p))
                 break;                 /* argument is finished */
 
             if (*p == '"' || (*p == '\\' && backslash_special)) {
diff --git a/windows/window.c b/windows/window.c
index 9f666d47..21507829 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -863,7 +863,7 @@ char *handle_restrict_acl_cmdline_prefix(char *p)
      * pointer past the prefix. Returns the updated pointer (whether
      * it moved or not).
      */
-    while (*p && isspace(*p))
+    while (*p && isspace((unsigned char)*p))
         p++;
     if (*p == '&' && p[1] == 'R' &&
         (!p[2] || p[2] == '@' || p[2] == '&')) {