diff --git a/src/basic/string-util.h b/src/basic/string-util.h
index a032ee8d6a..b5da8bea05 100644
--- a/src/basic/string-util.h
+++ b/src/basic/string-util.h
@@ -8,6 +8,7 @@
 #include "alloc-util.h"
 #include "macro.h"
 #include "string-util-fundamental.h"
+#include "utf8.h"
 
 /* What is interpreted as whitespace? */
 #define WHITESPACE          " \t\n\r"
@@ -234,6 +235,9 @@ static inline int strdup_to(char **ret, const char *src) {
 }
 
 bool string_is_safe(const char *p) _pure_;
+static inline bool string_is_safe_ascii(const char *p) {
+        return ascii_is_valid(p) && string_is_safe(p);
+}
 
 DISABLE_WARNING_STRINGOP_TRUNCATION;
 static inline void strncpy_exact(char *buf, const char *src, size_t buf_len) {
diff --git a/src/basic/utf8.c b/src/basic/utf8.c
index 15deef1ffd..4d1b31f26d 100644
--- a/src/basic/utf8.c
+++ b/src/basic/utf8.c
@@ -130,24 +130,24 @@ bool utf8_is_printable_newline(const char* str, size_t length, bool allow_newlin
         return true;
 }
 
-char *utf8_is_valid_n(const char *str, size_t len_bytes) {
+char* utf8_is_valid_n(const char *str, size_t len_bytes) {
         /* Check if the string is composed of valid utf8 characters. If length len_bytes is given, stop after
          * len_bytes. Otherwise, stop at NUL. */
 
         assert(str);
 
-        for (const char *p = str; len_bytes != SIZE_MAX ? (size_t) (p - str) < len_bytes : *p != '\0'; ) {
+        for (size_t i = 0; len_bytes != SIZE_MAX ? i < len_bytes : str[i] != '\0'; ) {
                 int len;
 
-                if (_unlikely_(*p == '\0') && len_bytes != SIZE_MAX)
+                if (_unlikely_(str[i] == '\0'))
                         return NULL; /* embedded NUL */
 
-                len = utf8_encoded_valid_unichar(p,
-                                                 len_bytes != SIZE_MAX ? len_bytes - (p - str) : SIZE_MAX);
+                len = utf8_encoded_valid_unichar(str + i,
+                                                 len_bytes != SIZE_MAX ? len_bytes - i : SIZE_MAX);
                 if (_unlikely_(len < 0))
                         return NULL; /* invalid character */
 
-                p += len;
+                i += len;
         }
 
         return (char*) str;
@@ -271,27 +271,14 @@ char *utf8_escape_non_printable_full(const char *str, size_t console_width, bool
         return str_realloc(p);
 }
 
-char *ascii_is_valid(const char *str) {
-        /* Check whether the string consists of valid ASCII bytes,
-         * i.e values between 0 and 127, inclusive. */
+char* ascii_is_valid_n(const char *str, size_t len) {
+        /* Check whether the string consists of valid ASCII bytes, i.e values between 1 and 127, inclusive.
+         * Stops at len, or NUL byte if len is SIZE_MAX. */
 
         assert(str);
 
-        for (const char *p = str; *p; p++)
-                if ((unsigned char) *p >= 128)
-                        return NULL;
-
-        return (char*) str;
-}
-
-char *ascii_is_valid_n(const char *str, size_t len) {
-        /* Very similar to ascii_is_valid(), but checks exactly len
-         * bytes and rejects any NULs in that range. */
-
-        assert(str);
-
-        for (size_t i = 0; i < len; i++)
-                if ((unsigned char) str[i] >= 128 || str[i] == 0)
+        for (size_t i = 0; len != SIZE_MAX ? i < len : str[i] != '\0'; i++)
+                if ((unsigned char) str[i] >= 128 || str[i] == '\0')
                         return NULL;
 
         return (char*) str;
diff --git a/src/basic/utf8.h b/src/basic/utf8.h
index 962312c5fb..301c50cc2f 100644
--- a/src/basic/utf8.h
+++ b/src/basic/utf8.h
@@ -14,12 +14,15 @@
 
 bool unichar_is_valid(char32_t c);
 
-char *utf8_is_valid_n(const char *str, size_t len_bytes) _pure_;
-static inline char *utf8_is_valid(const char *s) {
-        return utf8_is_valid_n(s, SIZE_MAX);
+char* utf8_is_valid_n(const char *str, size_t len_bytes) _pure_;
+static inline char* utf8_is_valid(const char *str) {
+        return utf8_is_valid_n(str, SIZE_MAX);
+}
+
+char* ascii_is_valid_n(const char *str, size_t len) _pure_;
+static inline char* ascii_is_valid(const char *str) {
+        return ascii_is_valid_n(str, SIZE_MAX);
 }
-char *ascii_is_valid(const char *s) _pure_;
-char *ascii_is_valid_n(const char *str, size_t len);
 
 int utf8_to_ascii(const char *str, char replacement_char, char **ret);
 
diff --git a/src/libsystemd-network/sd-dhcp-server.c b/src/libsystemd-network/sd-dhcp-server.c
index 4967f066dc..25b4cafaa5 100644
--- a/src/libsystemd-network/sd-dhcp-server.c
+++ b/src/libsystemd-network/sd-dhcp-server.c
@@ -271,7 +271,7 @@ int sd_dhcp_server_set_boot_server_name(sd_dhcp_server *server, const char *name
 int sd_dhcp_server_set_boot_filename(sd_dhcp_server *server, const char *filename) {
         assert_return(server, -EINVAL);
 
-        if (filename && (!string_is_safe(filename) || !ascii_is_valid(filename)))
+        if (filename && !string_is_safe_ascii(filename))
                 return -EINVAL;
 
         return free_and_strdup(&server->boot_filename, filename);